Butt joints on both sides on the same piece of wood

I think a much more common approach would be to make the upper cabinet a stand-alone unit with its own bottom. Then, when you assemble the two units, you'd use short screws driven up through the top of the lower unit.

This would have a secondary benefit as well. As you currently envisage it, getting the doors right on the upper unit will be a very, very tricky (and probably frustrating) exercise in fitting. If the upper unit leans even a tiny bit forward, the doors are going to make contact with the top of the lower unit when they open - and with 15 inch doors that's not going to take much. Going to a separate upper unit would allow a face piece to fill in a small gap (say, 1/8 to 1/4) under the doors. This would give you a bit of clearance to allow the doors to swing freely.