Team Game merged subs

So, I just finished a game on the 4 player team map small island peninsula (found here http://awbw.amarriner.com/prevmaps.php?maps_id=20757), and I was reviewing the replay. Something I thought i noticed while playing I confirmed. At some point in the game (around day 23 as far as I can tell) our two subs merged. That is, they were able to be on the same square, and attack separately. After the Cobalt Ice sub moves on day 25, this ends, but I found it very odd. Does anyone know what happened here? Both subs were submerged, which may have an affect.

This is turn 94 (day 24). If you have animation on, you can see another sub bobbing behind the cobalt ice submarine in the center, adjacent to the 1hp BH battleship.
http://awbw.amarriner.com/replay.php?games_id=218243&ndx=93
You can see that they work separately through turns 93-95, as after my (GE) turn, where the battleship is damaged/destroyed through both team members turns.

Just found it odd/interesting, if anyone has any idea how it happened?

I'm not sure how it happened, but I can tell you that this has certainly happened before. I've found that any team hidden unit can sometimes glitch and allow you to land directly on top of it. In that event, both units would receive identical damage from an attacker but I'm not sure what happens if the attacker is only allowed to hit one of the two unit types (such as a battleship attacking a cruiser/submerged sub overlap).

I've also seen a similar scenario in FoW games where the blocking function doesn't work and instead of being stopped by an enemy unit hiding the fog, you end up on top of it. I think I've only experienced that when at least one of the units was an air unit (more commonly both). Damage has been shared in these scenarios too, even though one is your own (or teammate's) unit. This doesn't happen nearly as frequently to me as the one with subs (and stealths) but it's definitely occurred more than once.