they combine wonderfully. It makes MXE the perfect drug, really takes the edge or anybody load off and makes the experience physically feel totally normal, while retaining the visual and euphoric aspects. Truly one of my favorite combos, I prefer it to mxe by itself
Benzos can make dissociatives feel more relaxed and comfortable but keep in mind that they also tend to reduce the dissociative effects at least somewhat, so if you want the most dissociation possible it might not be the best combo. If you just want to chill, relax, and have fun, you'll probably have a great time though.
Personal matter I guess, depends on what kind of feeling or experience you prefer. I'd say that benzo's on dissociatives are a waste of dissociative for the reason JG gave.