two circuits in one box ?
Assuming you mean the switchbox behind the receptacle... as long as it is big enough, this is fine. What's 'big enough'?
If you mean "replace a single gang box that has a duplex outlet with a new double gang box with an outlet and a switch", then you won't likely have a problem with 'box fill'.
If you mean "replace a duplex outlet with a combo switch/recepacle", AND your box is already tightly crowded, then you could easily have a problem with both physically getting the wires all back in and a thing known as 'box fill'.
If you have too many wires in a small space you are likely to have a problem with pinching the wires, either damaging them or the insulation, which can show up immediately or sometime later down the line. Also, if you cram too many wires in too small a space, then those wires can overheat when you are using both the outlet and the lights.