I am currently employed as an intern at a start-up, and I really like the work I do. It is very relaxed, low stress, but I just got a paid job.

I am a programmer/web developer. My new job isn't as challenging as my internship, but it pas well. Whenever I work at my internship I always learn new things, and I only work there 1-2 days a week, tops.

Is it crazy to keep working as an intern if I have a paid job? Even if I like doing it, and I learn a lot in the process? And if I should quit, how should I go about doing that in such a way that,

A) They would want to hire me back when they have the funding
B) We would be on the best terms possible