I found it easier to play with two bass drums. I had a better sense of control on two single pedals rather than one with two beaters and a slave attachment. It's just a little bit harder to get the spring tension even for both the master and slave on a twin pedal, and if that isn't at least close to perfect and you use about equal power with both legs, it's harder to get a more even/solid sound out, if you know what I mean. Also, the beaters on a twin pedal seem to "recoil" a bit faster than on two single pedals for some reason...Although it may just be me, who knows.
But a twin pedal really is cheaper, and saves you the hassle of getting two bass drums tuned exactly the same. I think it's just a personal preference of a lot of drummers who play with two bass drums, just as I explained it, in addition to the fact that it looks really cool! Lol