Some scribbled variations on the Upend idea.
The hinge could be either integrated in the robots, or the connection between the robots. I think I prefer the former, because a hinge could make the connection weak.
Falling and landing
A symmetric construction avoids the problem of the robots landing on their back when falling. The robots could also be round (barrel-shaped) so they will – hopefully – automatically turn when landing on their sides.
Maybe the hinge could automatically block when the robot is upended, keeping it from keeling over? The robots could stick together on their flat sides. A connection in the gap (e.g. magnets or a slightly sticky surface) might also help to stabilise the tower.