Polyventurism
Adventuring with two or more people simultaneously. This may be with or without everyone involved's knowledge.

This has two perceived meanings:

1: That this applies to any "party", or adventuring group, or
2: That this applies only to an adventurer who takes along an assistant/parter or small team, and takes on another while the first is absent.

Examples of (2):

a. When a hero's assistant/partner or team is captured or needs saving, and they team up with someone else during that adventure.

b. When a hero is taken away from their assistant/partner or team and put in a strange environment (an unreachable dungeon or prison, the bad guys' secret lair, another plain, a different dimension, some other world, etc.) where they team up with someone while there.

c. When the hero and their assistant/partner or team mutually split ways, due to either a great opportunity to go back to their old worlds, for one or more to go on to greater things, or simply a disagreement or fight... and before they all realize that it's not worth it and come back together, the hero has already found at least one other person that they're already adventuring with.
1. "With the five of us together, nothing can stop us! Yay, polyventurism!"

2. "Wait, so I've come all this way, after leaving my dream job, fighting my way out of the fortress, and having to track you down, just to find out you've found someone to replace me? Never knew you were a polyventurous type."

2. "You've got to make a choice, hero! You can't have two assistants. Is it gonna be me, or her?"