I was thinking along the same lines. One thought I had was that the party could lose all their Fellowship dice for doing this as a way to make it costly so you wouldn’t even consider doing it against an Uruk Hai (against a cave troll it might be helpful!). But I also don’t want the group to feel like Voltron where they can just “join forces” and overcome any obstacle. It’s tricky to thread the needle. Maybe some foes are just beyond you.
Exactly! Some foes are too dangerous for you to tackle.
I really love the idea though. Maybe it could be a mechanic that is "unlocked" or gifted by the elves in the form of a special item. Or a team ability that is unlocked when a party shares a lot of adventures together. The same way that Samwise was able (against all odds) to defeat Shelob in order to protect Frodo. Really love the Voltron parallel (MMPR fan as growing up). As you said, it's tricky to thread the needle but the idea is really good an it deserves to be considered more.