Yeah, when the other guy shut it off for me on the sly, it came back on within 2 days I think. So we'll see. I'm certainly not having any work done on it if the light stays off.
I also went ahead and ponied up for a guy at justanswer(.com.) Basic freelance expert site, about $18 for a conversation as long as you need it to be, I've had success using them before. Anyway, he said yes, the catalytic converters in a Mazda MPV are in parallel, but if one is mostly plugged up, then it will cause exhaust to back up into the engine and thus be forced by pressure into the other one, leading to the "added strain" effect. He said it was kind of unlikely, since the parts are the same age, it's not like I have one terrible one and one that's fine, unless maybe one sustained damage at some point (which is possible, I did go over a really violent pothole over the summer that knocked my alignment way out of whack.)
He noted, however, that there are also two O2 sensors, and one wouldn't affect the other, so even if it is actually sensor issue we're still looking at two sensors going bad at the same time (which is pretty much the issue tw noted, except instead he accused me of not mentioning that the first O2 sensor must have been failing months ago.
) He said the only other possible thing that might cause these codes is an exhaust leak somewhere up the line that might be confusing both O2 sensors, though he said again this would generally cause a sensor code, not a converter code.