Probably the closest thing to "production" two-barrel markers are Palmer Pursuit's Nasty Typhoon (pump) and Nasty Hurricane (semi-auto) markers. To my knowledge, though, they were only done on-demand/special-order or as conversions to donor markers--there wasn't ever a factory production line cranking them out or anything. Not sure if Palmers is still making them, though, and even when they were, you were looking at like $800+ to get one.
From what I've heard, pretty much anything else was done as a one-off or limited-run custom. (Didn't someone on the boards here do a triple-Ion, once? And I've seen photos of various double-Cockers, including Doc's, and a gold-finish double-Mag, too.)
As for "is there any real point", the answer is "mostly for show". Many/most fields don't allow markers that fire more than one ball per trigger pull, and some have bps rules as well, since there's no reason to hit another player more than once. Plus, they're significantly bigger and heavier than a comparable single-barrel marker, which is another strike against them. Add in the fact that they're also significantly more expensive, and you get something of a showpiece/white-elephant marker which looks great but doesn't typically hit the field much.
That said, if I had that $10 million others were fantasizing with earlier, I'm sure some of it might find a way to turn itself into a dual-Xmag for me... ^_~