The seats on Splash Mountain are also meant to hold two people, but as a solo traveler, I've never been forced to ride sitting next to a stranger. (I can't say about BTMR because it's been too long since I've ridden it.)
When I was a solo traveller back in 2011, I always rode with other single adults on every coaster - BTMRR, Everest, etc... Nobody "forced" me, and, in fact, when they directed me to that seat, I suppose I could have hung back and waited for the next train. But I always just said, "You don't mind, do you?" and just climbed in, the way I figured I was supposed to.
They never did mind, and in fact I had an awesome time riding coasters with strangers! I particularly remember one gentleman who couldn't remember Everest any better than I could - the backwards part took us both by surprise!
Why should I make the people behind me in line wait that much longer, if I don't have any issues riding with someone else? If it bothers you, don't get in, tell the cast member you need to ride alone, or even just block the seat so someone like me can't climb in next to you.
Disney doesn't have anything even remotely similar to the log flume ride at Universal, anyway, when I got stuffed between a lovely foreign gentleman's knees. I didn't know where to put my elbows!
He was very gallant about it all, however.