I want to dismiss it as bullshit too... but I met a couple of twins in Chicago, when I first came out, who were a couple, and open about it. Now it's possible they were just messing with the heads of all the guys who wanted to do 'em both, but... they seemed like the (un)real deal. Lost touch with them when I moved away to go to college, and don't know where they are now, or what happened to 'em. For all I know they could've written that letter to Prudie. (And if that's the case... I'm hurt. You guys couldn't write to me?) But I agree with Gawker: Prude gave these boys, assuming they exist, the right advice.
And remember: every letter that appears in an advice is just a good hypothetical situation to all readers save one. Or, in this case, two.