My apologies, I misread the post. I thought the question was if one could collab with two vocalists in two separate songs, not provide vocals for multiple people. I don't think there's an issue with providing vocals for multiple songs.
Commencing thank you in 3... 2... 1... *inhale* THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU THANK YOU!!! you just maae choosing between 2 friends SO MUCH easier