Would both jobs give you set shifts (that don't overlap) or is it a case of "ill give you whatever shift suits the company as long as its in your availability"? As long as having the two wouldn't impede on your ability to work the other I don't see a problem working the two during summer (I wouldn't recommend 20 hrs whilst studying but I gather one contract would expire by then)
Check the contracts before though, some jobs don't allow you to work a second job in x sector, require you to get permission to start a second job or have days/times you have to be free according to your avaliability.