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.