Connecting Cabs to RV50

[ckyratm]

Hey, sorry if there is already a topic on this but heres my question:

I just bought a Rockerverb 50 and am playing it through a Marshall MG412A 120 watt, 8 ohm 4x12 cab; I;m looking to purchase another cab soon and was wondering why it says in the manual not to connect two 8 ohm cabs. Also, if I want to connect my head to an 8 ohm cab and a 16 ohm cab, can I connect them to the corresponding outputs?

Thanks!