Is it just a feature of Tactician mode or a bug? If both of my source hunters fall in combat, even if my companions are still alive I get a game over. I can't seem to find specific information on Tactician mode other than "Enemies do different stuff, and there are more of them."

That's how my DOS games worked. If the source hunters are both dead, its game over.

Keep one alive!

Meh, that's pretty lame tbh. I don't see the point of having a whole party of people if you're wiped when just two of them die. You should at least be given a chance to revive your main chars via timer or something similar.

Never played Baldur's Gate, have you? :P

I don't like this concept either. It feels odd that game is over even if half of your party is still around doing just fine.

It might have something to do with scripting.
(for instance some dialogues trigger automatically between main heroes and other characters, and a crash might occur if both your main heroes are dead at that point.

But I agree that your companions should be allowed to continue fighting provided at least one main hero is ressurrected before the combat is over.