I have wondered this as well for some time. I tried san pedro and mdma. The mdma was taken well into the experience ( 6 or 8 hours or so,) and I had some LSD with full intentions of taking that at the same time as the mdma, but didn't. The mdma did not add much to the experience at all; it seemed to sober me up a little almost, with a slight rolling feeling. I never bothered with the LSD. I came to the realization that mescaline is perfect on its own. Why bother adulterating it? It has such a full, well rounded, gentle experience there is nothing that could make it better to me. I really like all three of these things, even candy flipping, but I feel mescaline is just too gd perfect, and shouldn't be combined with anything.
Just to show you how strongly I feel about this, I haven't posted here since I signed up, but this topic made me want to voice my words. And it was a ISO extraction from dried powder, for the cactus that time, and it was fairly strong dose. Two other parties with me agreed, and one was in a ++++. MDMA dosage was around 130mg then 80mg (forget what I weighed out)