As long as total load does not exceed 4 ohms, you will be fine. The amp has no way of "knowing" whether or not you have more than one cab, or that its a cab at all (it could be a four ohm light bulb for all it knows or cares!) All it sees is an electrical load. I'm not familiar with that particular amp, but I wouldnt recommend running both a 4 ohm and an 8 ohm load at the same time, as the two jacks may simply be wired in parallel.