I'm pretty certain everyone who dives side-mount, and plenty of technical-divers have zero problem with regulator switching at any depth.
In fact, I'd recommend people switch regulators more often. We all learn how to do it in our initial open-water class. By doing it semi-frequently, you'll be more practiced, know your regulators are properly stored/retained, and know they both work well (including at depth). It's a really easy skill, though I'd say if someone hasn't practiced it in a while, do some practice at the surface, face down, switching regulators.
The "DM" was apparently not familiar with independent doubles. The ignorance of other divers isn't a reason for me to change my kit. I simply mean "ignorance" according to it's definition, and not an insult, we were all ignorant about everything scuba at some point. The one caveat to the above, is that you need to ensure your dive-buddy is familiar enough with your system in order to air-share.
Anyway, the setup is clever, and interesting. I don't hate it, though I don't think I'd dive it myself, just because I've gotten into side-mount. For the general public, I'd recommend just going with two independent regulator-sets (side or back-mount), and switching, if you want fully independent redundant air and more air. Or manifolded doubles, if you just want more air. Or a pony-bottle if you only need redundant air.