Unless you have a room with near-perfect acoustics (ie. an arena) then your mix is going to sound like s*** if you do simultaneous recording as you're going to have some nasty bleed + reverb going on.
Unless you've got decent mics / cabs then I wouldn't suggest micing a cab unless you know how - DI on your mixer is preferable for this.
Using a 1/4' splitter will allow you to put both guitars into one amp, but it's going to sound horrible - especially if you've got distortion going on.
If you're serious about recording your stuff then I'd invest in a multitrack firewire audio interface like
this.
I'm not a pro unfortunately, but I found myself in a similar position to you and decided to go this route and the interface/mixer is working like a dream. If you want any more help then drop me a line