I was always told you can't stand and sit at the same time!
To burn fat is said to work harder and or eat less I do both at the same time and to gain you must work harder and eat more. I'm no expert but for me it may look like I have gained mass while burning but the truth is I think it's just the muscles being put into shape and the muscle memory that sets in. Losing and Gaining at the same time sounds like a fish out of water to me!
Even if your looking pumped up after a workout and it stays for a few days your not nurturing the muscle enough to make it stay, this is just my thoughts and not a soild fact.