Just what happens when two figures are overlapping and a guard action interrupts, or a trap interrupts, to target that square?

The only trap type that is capable of catching two heros in one space are the door traps - if they deal damage, all heros are affected, if they end the turn, the OL moves the active player figure to an adjacent empty space of his choice.

If the heros catch two mobs on one space with a Guard attack, both monsters are hit with the one attack.

See the FAQ and/or the GLoAQ thread (first forum page), it is clarified there.