Many people try sticking two trips in one and I have done so a couple of times. However, when it comes to Mexico and Guatemala/Belize I wouldn't advise it. You would need at least 3 weeks in either place to really enjoy it so you'd want to decide between 3 weeks in Mexico, or 3 weeks in Guatemala/Belize. Remember that the attractions in these places aren't just the city centers, but the villages and towns you can visit in the vicinity.
I've never been a fan of packing in too much on one trip. All you would be doing is traveling from place to place on buses or airport transfers. Unless I was terminally ill maybe then, but otherwise I prefer to take in as much as I can and relax too. What's the point of going traveling and not relaxing?