First off, recrding 2 drums at once would be impossible really.. unless your a robot who can play the exact same thing, with the exact same timing (to the millisecond) and never make one little mistake + there is more reasons but it doesn't matter.
Anyway, do you realize that you can record a full kit with only 2 mics? and actually sometimes even one!
Ive done this a good few times and it works great, and im talking about with bands who come into the studio recording for an EP they put out.
I hear some of the bigger producers do this too sometimes.
And don't go with the split cable, that would be stupid, especially if your using condensers with need phantom power.
Second option would be to buy a mixer with the amount of spaces you need and then plug it into your mbox using the left and right (stereo channels) of the mixer, this will give you a live mix but wont leave much room for edits in protools (im guessing thats what your using) only with the 2 channels.
Other method would be to buy an interface that accepts more inputs, i guess a presonus FP10 or maybe something smaller, say another interface with 2 channels running at the same time as the mbox.
Really, it depends on what your going to be recording in the long run.. there is no point buying something to do the job just now and then in 5 months you need an even bigger or different interface/mixer costing more money.
What is your current mic situation anyway?