This one will hopefully be useful to you guys. Its a mixer that can run in series and parallel with a blend control as well. I don't know too much about it but saw the build doc on Facebook.
Sounds like you can put it in a dual effect box or wire it up with switched jack to lend other effects together
Heres a link to the build doc (hopefully):
https://lookaside.fbsbx.com/file/Phantom%20Pain%20Paramix.pdf?token=AWzrv_675Px5syjjCeAmdWF5Q5HouV6u0oIgcF3taj1fzaFgCLBSaVD0AnFKC2V4iPkXqa_r02dnpPTZMpUamBbal2mi8d3Uia3WdhLjGCdU5fbCm7T6u4M_AEyduknZZ8va1e2RwMXb6kCmCzlPyI4J"send/return A is always in parallel with send/return B. Send/return C is always in series with effect B. The blend knob allows for effect A to stay in parallel with effects B and C, mixes the parallel A and B into effect C, or a combination of series and parallel."