As Chief pointed out, "superposed" simply means one barrel on top of the other. Browning happened to adopt the word as the name of their OU's. But in fact, all OU's are superposed.

I had a Laurona OU with the double/single (or twin single) triggers. Pull the front one, lower barrel fires first; pull again, upper barrel fires. Reverse for the rear trigger. Or just use as a DT. Interesting concept.