Yeah. This gets re-hashed every few years. It's a valid technique, but not necessarily any easier than just making sure your masks line up when etching a double sided board. It's all here:
layers of a PWB (PCB)
layers of a PWB (PCB)
Message
Re: Gluing 2 single sided boards to make a double sided board.
2015-07-06 by jamesmichaelnewton@...