Sew them together
Also I can't vouch for the skin top wigs with big part sections, I don't own one of those so they too may not have much hair pointing towards the back.
Here's an examples of a wig which hasn't been layered. Wig fever and wig fashion have loads. Most suppliers will tell you whether it's a skin top or matted top wig if you ask.
This is the main red wig. The front part of the wig has been cut out and the bangs of a black wig has been stitched in place.