https://swmgamers.com/SWM/Rules/ResolvingEffects.htmlI read it this way...
Step 2 - Declare a target. At this time, you declare a target and make the save. If you make the save, Dovin Basal has no effect. If you fail the save, then you either agree to combine fire or the target attempt fails.
Step 3 - If Dovin Basal canceled the targeting, you never reach this step. If you made the save or agreed to combine fire then resolve effects that trigger when targeted.
Step 5 - Declare the attack, including combined fire. If you agreed to combine fire back in step 2, then you must combine fire in this step.
I'm about 80% confident in that reading, but let gandalf make the official ruling.