So I had Desperate Hour as our finale and we ended up with this scenario: Diana Passil was the only standing Hero with 1 wound left, adjacent to General Weiss who had 2 wounds left. At the end of the round, the Imperial player used Hidden Detonators and killed both of them.
Do you guys think that it was a tie? or someone actually won the campaign?
sounds like a tie to me 
Following the end of mission goals, I would say it's a tie. But if I was the Imperial player and I pulled this stunt, I'd point to the RRG, pg. 9:
If multiple mission rules would resolve at the same time, the Imperial player decides the order of resolution.
Now I don't think that rule is really useful in this circumstance, but it's the closest thing I can find.
Additionally, if you read the debriefing info for both the win and loss conditions, it states:
The rebel fleet overhead is either destroyed or escapes before the main Imperial fleet arrives
So going on that, I would say it's a victory for the Empire.
Well, technically you win the instant one of the winning conditions is fullfilled.
So with hidden detenators you choose Weis , roll the die , diala takes dmg and dies. Winning condition. You wouldn't get to the next step in which Weis dies.
This sounds like the most Imperial of victories to me!