If one of the sides of the cathedral is the under side of the roof, you'll need something that will flex without cracking in the corner. If the cathedral is a "structure" that projects up into the attic space with no connectivity to the roof, this is less of an issue.
The den in my home has a 9-1/2' ceiling that intersects with the roof along the outside edge of the house, angling down to where it meets the outside wall at the typical 8' height. The seam where the angle met the flat was forever cracking. I'd spackle it with flexible caulk and repaint, and it would crack again.
I solved the problem when I remodeled by using some flexible corner tape - I think it was the no-coat brand.
http://www.no-coat.com/
The concept of this stuff is that you mud an outside corner, then apply the tape into the mud with little or no extra mudding required, different than what's required when you use metal corner bead.
The tape is regular drywall tape that has a rigid plastic spine with a molded-in seam. I installed it along the seam on the ceiling backwards - with the side that would ordinarily face towards the wall facing out to the room. The seam creates a nice straight line, and I made sure I didn't put any mud into that area - just paint. 14+ years later, no cracks.
The way your sheets meet up, you've got a lot of mudding to do - I'd try sliding the already mounted sheet down to where the new sheet meets the old sheet at the outside edge.