I have that mod and it still doesn't seem to work for festival days at least not for me. Every other time seems to be working fine now, but just today I went to the spice festival with a co-worker and he wasn't available as a karaoke duet option.
So unless “on festival days” means “any time there’s a festival happening anywhere in any world” it’s not just on festival days. My Sim cannot duet with anyone in their group, even with nothing else going on and few other Sims present and the entire group standing around next to the karaoke machine.
