Has there ever been a situation where the following conditions applied:
At the same point in time
Ais actively war with
- other troops of both sides work together* against
*= not just "happen to target the same object" but "actively communicating or planning on each other's support".
The closest that comes to my mind was Germany and Russia when they took Poland but at this point they haven't been in a real war yet.
Another potential I am seeing is in the Syria conflicts as some groups stated they aren't even sure who they are fighting for/ against.
This is somewhat based on Has there ever been a truly multi-sided war? except for the "collaboration at the same time" part.