How is this one:
The way I have done it is to use scoring as a sort of AND condition. B52 enterign gives Side A a score of 2, B52 actually being detected (there IS a unit detected trigger in the event editor, not sure why you say there is not?) adds another 2 points = 4 points, 4 points score trigger triggers the mission.
Second bomber has a same setup but gives 6 points each, points reaches at least 11, second mission fires.
If the B1 showed up first he'd still launch his mission (reaches 11 points) but the B52 mission wouldnt launch anymore because score would never equal 4. Solutions: use an extra side for scoring per event, but that can rack up the sides and more sides = less performance (make sure to set a scoring only side to non playable and blind awareness level
Or, make a minus X score (where X is detection + entering area score combined) to reset the score. Problem might happen if events start running mixed through each other.
Other problem, separate one, is that as you can see in the demo, the second non target B1 gets attacked by the launched fighters for some reason, even though it is not a target of the mission and Engage targets of opportunity ROE is set to no. Not sure, might be a bug.
Let me know if this helps! If it is not clear ask, am in a bit of a hurry right now thats why the post is a bit rushed. Look at the event chains and let me know if it doesnt make sense.