Soldiers A and B can't heal each other using medi-kit, they get the message "There's nobody here".
Function ActionMenuState::btnActionMenuItemClick() checks that units are on the same "map level", it ignores the units heights and whether they stand on a "stair" block. This is probably going to be annoying on long slopes like mountains (TFTD had quite a few)
More strangely, A can't strike B with a stun rod with the same message, and if B tries to strike A he loses TUs but nothing happens.
In this case the first check is TileEngine::validMeleeRange(), and the actual hit, I don't know where it is.