its has 2 outputs at 4ohms each.. you can't run these cabs in series, they only have one jack. I've run them together plenty of times, but to be honest Since I got the Peavey, I just use that and leave the yamaha at the practice spot.
Even if a cab has two jacks they are more than likely wired in parallel. So even if you daisy chain them you're not running them in series.
If your drivers are 8ohms each and wired in parallel to make each cabinet 4ohms then what your amp will see is a 2ohm load.
I thought the MAG300 had one speakon and one 1/4" phone jack for speaker outputs? If so then it's meant to handle a 4ohm minimum load.
Some of their bigger amps have two power sections each with one speakon and one 1/4" phone jack. They WILL handle 4ohm loads on each side.
(NOTE: all of the above ohm ratings are nominal.)