So the sensible thing to do is to use one head per cab. Does not really matter which. Just make sure that you operate within safe parameters. The wattage rating of the speakers usually means how much power they can take before the voice coil melts, but they usually will suffer mechanical damage long before that. So use your ears. Increase the volume carefully and as soon as you hear any kind of farting or distortion from the cab, you're getting into dangerous territory. Back off a bit. Mind that turning up the bass knob usually will push more watts into the cab, too - so know the limits of your equipment and never cross them.
With that out of the way, you can use the line out (XLR) of one head into the FX return of the other head. unfortunately, Markbass is a bit off the mainstream on the FX loop, which is usually located between the preamp and the power amp. On Markbass amps, however, the FX return is affected by the EQ. You should get along by setting the receiving amp flat, though (all EQ controls at noon, all filters at zero).
So after you've set all that to your liking, you can use the master on each amp to set the desired volume of the cab. This should get quite loud.
Thank you very much for this comprehensive and easy to grasp answer.