You hit the nail on the head without realizing it.
Why can't you support two ranked queues?
The key word there is SUPPORT. There's barely enough supports or people who actually INTEND to play support (and not conscripted like the people who couldn't say "yasuo mid only or feed" fast enough) for one queue, let alone two; that's why your lobby times end up being longer than the game itself. Releasing Solo Queue, as they implied earlier today, might actually make the situation WORSE. They know they can't tell people explicitly "Play support more, you bastards!", so they're trying to push every other alternative as far as they'll go.
Funny enough, I think the issue with Dynamic Queue is causing the exact same problem that contributed about half a dozen nails into Dominion's coffin. With not enough players, the system tries to expand the range further into other ELOs and divisions to attempt to find someone, ANYONE, that will fill the required roles. If that's the truth, we're in some real deep shit.