Layout method using concrete retaining wall block for angled wall
When it comes to cutting block, you don't need to cut it (but cutting a score can be helpful). Using a rock hammer (and perhaps a chisel) to break the rock is the way it's done.

The method for shaping rocks is kind of simple, but a video is a better way to describe it... so here's a few youtube videos about shaping rocks.

Flagstone (good info)

Rock hammer (more like what you're working with)

With a chisel

Otherwise (if you are adamantly opposed to breaking or shaping stone), you could build a form and pour a concrete joint (like in the shape of a Y). If that's the plan, I would probably start with this, then lay the rock. 

Also, a suggestion to make it look a little better might be to make an exposed aggregate joint. There are several ways to accomplish that, but it's not really relevant here.