I have a similar issue, and here's how I've done it so far.  I have a Digitech S-Drum pedal, which is a drum machine you can program on the fly with your guitar.  It has a guitar in, guitar out, and stereo mixer out.  If you plug in just the guitar in/out, the drums come out the guitar output, eq'd for a guitar cab.  If you add a plug in the left mixer out, it gets mono drums and the guitar out just gets the dry guitar through.  Adding the right mixer out gives you stereo drums.
 
	 
 
	What I want is to get my processed guitar and drums out the stomp headphone port.   The best way I've figured so far is running the L/Mono drum output in to the right input of the stomp, and my guitar through the sdrum into the left input.  At the beginning of my chain, I split the left input to the top path and the right input to the bottom path.  I put the merge before the LA studio comp last in my patches (jason sadites style).  The compromise here is that the drums also go into the compressor, but I don't mind that too much; I just use it for practicing anyway.
 
	 
 
	I wish I could put in the split and merge blocks without actually putting one of my 6 precious blocks on path B, but that doesn't seem to be allowed.