Running two heads into one cabinet will be possible if the cab is capable of stereo. One head will go in the Left input, the other in the Right input.
Running two heads at once isn't really that succesful in my books. It never really worked the way I wanted since I don't really have a place where I could run my amps semi-loud, let alone two hi-gain, very powerful amp heads.
Also, what do you think is wrong with the EVH 5150 III and why don't you fully like it on it's own. It's probably good figuring that out since you'll be able to determine if settling for a second amp is really worth it anyway. Figuring out what you want from your amp is probably your next step. Then decide if you can get as close as you'd prefer with your existing setup and perhaps minor mods 'n maybe even some more serious mods.