how to cut roll roofing properly around outside corner?

There are stick down rubber products they will stretch around that corner without cutting.

but the cap sheet is not flexible like that

Good thing you don't get snow. What a clusterf***. Don't know what you mean by rolled roofing but that is a basic flashing detail for any flat roofer.

You need a metal flashing like the image. Best is copper sheet and all joints soldered. But next best would be aluminum held together with rivets and joints covered with eternabond tape. A good caulk (I use osi quad) could last a while but probably the joint will crack and open. This flashing must go under the stucco. Under the step flashing on both sloped parts and over the valley flashing downhill.
Temporary fix would be a tarp, not a roll roofing. Big enough tarp to cover both sides of the slopes. If you don't cover all of the roof there, corner flashing that is installed over the step flashings will leak.
Search for images and videos of roof flashing. Chimney and dormer flashings apply to your situation. All flashings overlap and if a low slope, joints must be sealed or caulked or other means against water traveling sideways and sometimes, could travel up slope if a valley or such.