You could do this, why would you? If I understand you correctly, you have an electrical panel in the room and there are 6 unused spaces in it. You want to use two of them, to power two seperate items, and put the receptacles in one box? I doubt that anyone would recommend this as being a less expensive way to power, like a couple of lights. You would need to use a deeper box to allow heat to disipate. It could also be dangerous for someone else that didn't realize there are two seperate circuits in an ordinary looking duplex receptacle and start to do work on it without turning off both switches.
I had installed several circuits in my space and have found that sometimes when you plug a timer into a duplex you can't plug anything else in there anyway. If you have both circuits in one duplex you may regret it when you try to plug things into it. VV
