If you want a key to be active on the main only, the only two choices I could see here are:

1. Bind the spell to a key on the main knows.

2. Amend the FTL DPS keymaps to not broadcast 2 to the followers (set it to All w/out Current).

I'd personally use option 1, as it'll be easier.

PS: Theoretically, you could turn off key-maps every time you wanted to do it, but I assume we're talking neat 'n easy solutions here, not painful ones... hehe.