If I understand your question correctly, you want to be able to make two or more stereo sub mixes to send out to be "processed" and then returned, you can do it with a mixer if it's equiped with sub groups. If you have a four or eight buss mixer, alls you have to do is assign the tracks you want to process to, say, subgroups one and two and another group of tracks to sub groups three and four and patch that to the device(or devices) of choice and then return it to the mixer either through aux returns or even two or four mixer channels.