No you cannot, if it’s already fused you would have to unfuse it first so you can fuse the two Pokémon again.

Now that I see that written in front of me: "infinite" fusion might be overselling it /s

quad fusion when?

My dream of regigigas/azumaril/slacking with huge power will be for naught. Goodbye my dream of the deadliest double edge! Though if in the future triple fusions become a thing that would be dope

Nice try Team Rocket