I suggest you hook it up yourself and find out. Unless someone has the same equipment and hooks it up they won't be able to tell you.
Personally, I would use the slave to drive one cab - which ever cab sounds the loudest so I can adjust it (down in volume) to roughly the same volume as the softer cab.
Probably won't be able to notice any great volume difference between driving both cabs with the head or driving one with the slave - although with the slave there should be more output capability (roughly 65% of the normal 4 ohm 150 watt output for each power amp).