I don't think it makes sense to evaporate it at this point. I bet you're going to lose much codeine if you heat it to evaporate, your solution should be slightly acidic now and codeine is not really stable for a very long time in acidic conditions when exposed to light, it's fairly easy oxidized at the benzylic carbon atom, higher temperature will only speed up this process, and evaporating water to dryness is going to take a hell of a lot of time. Not a good idea at all. You can raise pH of your solution to ~10 to precipitate freebase codeine which is much less soluble in water than codeine phosphate is, this would also make paracetamol more soluble at the same time (deprotonation of phenolic OH at basic pH). Then you collect the solid and dissolve it in a small amount of acidic water (citric acid will do). Alternatively, you could reduce solubility of codeine phosphate by adding a salt with common ion into the solution, e.g. sodium phosphate. After mixing thoroughly and refrigerating the solution, codeine phosphate should precipitate out of the solution.