Recently started a two-a-day approach for 3/5 of the workout days each week. Been very effective at mass building and simultaneous leaning (for the obvious reason that you're metabolism is jacked up for a much larger proportion of your waking hours). Couple of things to keep in mind:
1) You'll probably get better muscle results with a resistance/resistance approach rather than a resistance/cardio split each day.
2) You'll be subconsciously tempted to reduce your training intensity because, after all, you'll be back later in the day. Don't. Working twice as long at only 70% of your capacity won't build much muscle.
3) Train the same basic muscle groups in both sessions each day. But there's no reason to do the same routine. I like to do olympic lifting in a morning session followed by a more typical training split in the afternoon. If you're supersetting or using other approaches to maintain intensity over two sessions per day, you really shouldn't need to do as much cardio as before.
4) If you can make the time in a day for two training sessions, then you can make the time to add additional rest--ideally with a nap or two between training. You should also adjust the frequency of training each muscle group or chain to account for the increased daily volume. The intensity of demand on your central nervous system should also be taken into account, unless you're just doing simple, isolation, single-muscle exercises all the time.