Well, as far as i can tell, Boffs have never revived other Boffs in combat. Only afterwards. But perhaps i have missed the time, when they did so.
If you want a Boff to revive you doring a combat, you can press "Call for Help". I was myself not aware of this feature until i read about it in a loading screen.
I actualy just played on Holodeck yesterday, and a couple BOffs died in battle. I can confirm that my other BOffs did in fact revive their teammates during the battle/red alert period.
On Tribble, they never do. I had one of my engineers die, didn't realize it right away, left the area and found out that he was left dead 20 or so km (or m; I forget what they use) back. It's a big problem when only I revive people. Not cool.