For most twins, and especially if you're going C pack sized motors, you'll need to upgrade from the xt30 to the xt60. The number in the xt identifier is the number of amps it's rated for - you'll want that connector to be equal to or greater than the capacity of all the ESC's in the system added together.
However if you power your twin motors with separate batteries to their ESC's you might still get away with the smaller sized connectors - if those ESC's are 30 amp rated or lower.
Either way, make sure to disconnect one of the power leads going from the ESC to the receiver - otherwise you've got two separate BEC's pumping 5 volts each into the receiver, at which point the smoke comes out of it.