There is a single subwoofer trim and distance control, although there are two separate unbalanced outputs and one balanced output (which could be handy for folks using pro amps to drive passive subs or the folks using a Behringer Feedback Destroyer).
Just some thoughts off the top of my head... I'm not sure how your particular speakers are set up, but some people use the speaker level signal from the amp to drive speakers with powered subs, in which case you could simply set your surrounds to large (or to small with a 40Hz crossover, since many powered subs in speakers could still use a little help with the really low stuff) and use the two sub outputs to feed the front subs. Or you could get two Y-adapters and drive all four subs from those two outputs, and experiment with the phase setting on the subs' amps to see if the single distance setting works for you - it's very possible that it would.