Yep, two is the magic number I have found.
I liked the A4 & MD pair because they were so sonically different, they covered a lot of frequencies, tonalities. The drums cut through in a nice musical way.
Right now OTMKII and DN is my ideal pair because I just love the Digitone and the OT MKII gives me MOAR DIGITONE by sampling it, often in full 8 voice stereo unison. Together, I get 12 tracks of Digitone. And Scenes!!