Let your employers know that you have an appt that was made 6 mos ago. Dont have to say what it is. Let them know about it only if it will conflict with scheduled hours.
Go to the interview and see what its all about! Be firm with your questions, salary, hours, vacation, sick time and see if there were any lay-offs in the recent years. Dont worry about asking those questions, you have 2 jobs now so if they dont hire you, its their loss.
Grats btw, you might have 3 jobs... LOL