dramamama611 said: "why would you want a 2 parter? (Musical or otherwise) Im not against them, per se, but I dont think people SET OUT to create them."
I think the benefit of 2-parters (or 3-parters, as I've occasionally seen), is that it makes for an epic theatrical experience. You can really immerse yourself in another world, in others' stories for virtually a full day.
As for musicals:
I think part of the benefit of musical theatre as an art form is that it taps into deeper emotions, and a heightened sense of reality. So to see that done as an epic, all-day experience, has the potential to compound the effects of musical theatre; it could be all the more epic, all the more impactful, and all the more deep in emotion. I imagine the reason it hasn't been done is that it would just be so incredibly, monumentally difficult to achieve in the first place, and all the more difficult to actually do it well.
The closest thing we probably have to this in the MT canon would be the Falsettos trilogy, though I'm not sure if all 3 of them were ever performed together. Aside from that, the longest musical I've ever heard of was the first production of "Giant" at signature, which was supposedly about 4 hours long, before it was trimmed significantly to a more traditional length for its NYC run.