"Beaker, I might be barking up the wrong tree but would any of the existing loop/splitter/mixer circuits be workable for this?"Yes that is exactly what you want. If I was in the original poster's shoes I would just build two delays in two seperate boxes, and put them in two of the loops in a switcher/looper pedal.
I know it means having three separate pedals instead of one, but the extra versatility (you can add any other pedals you like into the loops) more than makes up for the space saved on your pedalboard.
You can build your own quite easily, but the ones by Bright Onion down in Brighton are superb quality, and extremely good value. For the price he does them for, it's hardly woth the effort of building your own in my opinion. And he will do bespoke work too. He made a custom order ABC-Y looper/switcher/router for my son's three amp rig a while ago, which is superb.
http://www.brightonion.co.uk/Ah, Johnny Fucking Marr! Yep, two Fender Twins at each end of a corridor with a mic halfway between the two, and an engineer sitting by each amp tweaking the trem rates to keep them in synch.