For this- the closest thing we have in the game is abilities like Mandalorian (Mira of Nar Shaddaa), Rebel (Saw Gerrera), or Yuuzhan Vong (Black Market Biotech Dealer). They all let a character count as another faction without losing its original "faction" (not really since Fringe isn't a faction, but lets them count as both Fringe and a faction). Affinity doesn't work like that because WOTC ruled it that way. Doesn't matter though- as long as everyone understands how the rules are supposed to work for this one, technicalities are irrelevant.

Submitted

When Rex and company landed on the far off planet, they had no idea that it held a pre-invasion vong force. During the battle, with both sides taking serious losses, a new enemy appeared, forcing the surviving commandos to team up with the few remaining vong, to defeat the new enemy.

A long time ago, in a galaxy far, far away...

The Rebels have secured the Death Star plans! In a desperate fight over the planet of Scarif, Admiral Raddus dispatches his best fighters to recruit additional pilots. The Rebel fighters find a remote enclave, where an aging Revan leads a group of pilots of the Old Republic. After explaining the danger posed by the evil Death Star, the Rebels secure Revan's agreement to assist in the charge. On arriving at the battle, the pilots find disaster, destruction, and, most improbably...hope.

A long time ago in a galaxy far far away. *cue thematic music*

Broken! Dark shadows fall across the galaxy as the dark and sinister Bane has opened a portal creating a rift in the space time continuum. Jedi from all eras of time are being stripped of their powers and encased in dark side energy cages. Very few Jedi have managed to elude the grasp of Bane. Mace Windu and Aayla Secura have traveled back a thousand years to battle the forces of evil. They are joined by 2 Jedi Knights of old Revan and Malak. All have one mission. To restore the balance and fix the tear.

Did I just hear you correctly Grand Admiral?

Yes, General Weir, you heard correctly. My proposal is combining expertise with a specialist force of the Rebel Alliance.

But sir, they are the enemy! TRAITORS to the empire!

Your anger does not suit you here, General. Survival of this onslaught is the paramount priority. If we must, temporarily, be assisted by the infidels, so be it. Should we prevail, we will have the advantage of retaining information from their tactics that can be used against them in a later encounter. If we fail, we will perish. As I see it - neither side has much choice in the matter.

We have all the expertise we need on the lawful side of the Galactic Empire. These criminals add nothing. If we cannot succeed, no-one can.

If only that were so, General. In analysis of historical art of the incoming opposition forces, one thing is clear. We have to expect the unexpected, while at the same time, give them something they may not expect.

You think they haven't thought of us combining forces in this way?

Of course they have. Say what you will about who we will face, but mark my words - there is nary a fool amongst them.

So what is the plan?

All contingencies are impossible in an operation such as this, so flexibility is of the utmost importance. Defensive abilities are too innumerable to ensure reliable counters. Attack and damage output will reach unheard of levels. Their movement abilities will seem beyond comprehension.

So how can we prevail against such unknown powers?

By being unknown ourselves. In an encounter such as this, it comes down to kill or be killed, as brutish as that sounds. We must inflict mass amounts of damage as quickly as possible.

Now you're talking my language.

It serves a purpose General. A means to an end, not merely random carnage. The key is that then this will be followed by extreme mobility. The best way to prevail against an opposition with immeasurable power is quite simply to not be attacked.

How do we do that?

You provide expertise with speeder bikes, and the Rebellion's General Crix Madine is a master with commandos. Combining forces helps us to to be doubly effective with the Alliance's Commandos on Speeders. The famous rebellion heroes Luke and Leia will be in commando gear on a speeder together. We know that Skywalker is force sensitive and Organa has unrivaled diplomatic savvy, but I also sense a deeper connection between the two that makes them a force to be reckoned with. They will be aided by an additional solo rebel commando on speeder for quick relocation. My tactical mastery combines with Madine's cunning to help us get powerful first strikes, but I also have ways of helping them be opportunistic on later strikes as well. I can position forces for both offensive and defensive purposes. The Chagrian politician helps keep our communication strong, something the rebellion has never had the advantage of, with their survival necessitated on being covert. Besides, his skin is the most beautiful shade - don't you agree?

Sir?

Furthermore, the rebel's golden protocol droid can help cloak our location, while also expanding our communications, and even makes our BB unit more effective. The BB unit combined with an elite commando on foot will help secure our tactical advantage on the ground with the location's innate defenses. The elite commando also has a special surprise waiting if the opposition chooses to bring droids or mounted weapons to the fray.

But what of the disgusting furry little beast?

The EWOK, General, is a Princess of her tribe. She is more special than you'll ever know.

When do we start Grand Admiral?

It has already begun. You will find the Rebel General Crix Madine waiting in your quarters, ready to discuss the cross training of our units.

He's in my quarters!?!

Yes General, and I won't hear a word of complaint from you about it, lest you desire a demotion.

Yes sir.

And Weir, one more thing.

Sir?

When we do prevail, if you have performed admirably . . . I'll allow you to have a post-op debriefing, so to speak, with the rebels. We must ensure our tactics are not retained by the Alliance. You may use any means at your disposal to ensure that. I'm sure you will be . . . thorough

YES SIR! UNDERSTOOD SIR!

There are no guarantees on this mission, so take every moment to ready yourself and the team for the unexpected. Dismissed.

[for reference: The Princess Bride]
So it seemed that the Empire and the Rebellion were at war. Well, it turns out that they were only mostly at war. There's a big difference between mostly at war and all at war. Mostly at war is slightly at peace. And all at war...well, with all at war there's usually only one thing you can do: blow up the other guy's space station.

General Weir had never really been the same ever since Darth Vader fired him. Same with his friend, Captain Sarkli. However, one day they happened to run across Luke and Leia on their speeder as they were finishing their paper route for the Daily Rebel. The headline of the newspaper that day was, "Darth Vader might be our father, but he isn't a very good one." This headline piqued the interest of the disrupted commanders, and so they invited the twins to join them for a conversation over bloomilk in the Death Star Cantina. The twins let it slip that they were planning to storm Vader's castle to steal his very rare collection of antiquated Dad Jokes (the ones mounted on square bases). The ousted imperial commanders felt the old cunning returning to them once more! Being the opportunists that they were, they joined with the twins in their quest to disrupt their mutual enemy.

As the twins flew off in their speeder, Weir called out, "Have fun strafin' da castle!"
"Do ya think it'll work?" ask Sarkli.
"It'll take a miracle!" said Weir.