This combo works well on three levels. To begin with, a dedicated CY/BMU early game generally can produce an early Province or two, which is all-important in a Tournament match. Secondly, once the Province has been added and a Tournament bought, it becomes easier to match them up because of the return one card rule of CY. The third reason these cards work really well together is that you can chain Provinces over turns to keep your opponent from being able to use his tournaments. This can utterly destroy their tempo in the end game.
Note that with any CY strategy it is okay, sometimes much better, to add multiple CYs. They stack pretty well because if two are drawn in a hand one can be saved for the next turn. You can actually set up a cheap and fast engine this way. It is inferior to Masquerade and will get beat on some boards but it really isn't a bad way to rush.