Don't say anything until you know for sure you've got the other job, but your best to be up front and honest about your second job asap with your current manager. Stress that you consider it a secondary job and that your current job remains your priority (at least at the moment - you don't need to clarify anything more than that until the other job becomes a possibility for permanency). As long as you don;t start to slack at your current job and can maintain a balance with both jobs and your personal life, I find that most managers can appreciate that they have someone who is willing to do whatever it takes to get by and work their *** off.