If our muses haven’t fused in canon, send “Fuse with me!” and I’ll tell you about them!
So a Pearl fusing with a Pearl, just like any other same-gem fusions, makes a Pearl. But Holly isn’t a normal Pearl; as far I understand it, she’s an amalgam of a sentient Pearl hologram, a bronze Pearl, and a Labradorite (and a little human, but that doesn’t really come into play here, the point is that she’s multiple gems, so the fusion doesn’t have to be a Pearl).
I think their fusion would be Apophyllite, a gem said to be extremely helpful for communicating with spirits. It also has that nice light blue, semi-translucent color. I think she would be a little under 2 Pearls tall.
Her personality is a bit trickier, because a fusion is more than just an amalgam of the components’ traits. It’s a relationship, and although we’ve RPed a bit, it’s still unclear exactly how my Pearl and your Holly mesh. That’s part of the reason I don’t think their fusion would be very stable. After looking over some of your RPs and muse info I could find, it also seems like Holly doesn’t have the best self-esteem, and can hear (and speak with?) ghosts.
Given Pearl’s history, that could end badly, she’s still dealing with enough metaphorical ghosts. I feel like, for however a short time Apophyllite managed to stay together, she would be soft-spoken, anxious, and easily spooked.