Generally it shouldn't make a difference either way. The sockets on the back of amps are [i]usually[/i] connected in parallel, as are the sockets on the back of speaker cabinets. Therefore, either way, the amp is seeing a 4Ω load (if you're using 2x 8Ω cabinets). I would usually connect each one to a separate socket on the amp, if you have the choice, since there is an argument that there will be slight losses in the speaker leads, so by using two direct to the amp you're minimising the power lost.
The exception is some valve amps, which have different sockets for different impedance taps from the output transformer (edit: as chris_b mentioned). In those cases, you should probably only use one socket, but you have to make sure you connect them to the right one (e.g. two 8Ω cabs would be connected to the 4Ω socket, etc).
[quote name='Beedster' post='765989' date='Mar 6 2010, 12:49 PM']The instructions for my Ampeg Micro/SVT210AVs state that to run two cabs they muct be daisy chained, this despite the fact that the amp has two speaker outs, which kinda begs the questions of why there are two outputs? If anyone out there is answering Claky's question could they shed some light on mine also (sorry Clarky).
Cheers[/quote]
Just had a look at the manuals on the Ampeg site, and I see no mention of that...