Hey Kat,
Make sure you have a very clear understanding going into whatever wwoof/farm internship you are considering. I personally always had the farm internship be my full time job - sometimes to the tune of 55 or 60 hours a week but there where often folks who would do a split between farm work and 'real' work. Most often this was one half of a couple. It was pretty common for these people to burn out totally - that kind of schedule can be very very draining. I think it has to do with the direct light exposure tricking the brain - especially in conjunction with more typically behind the counter sort of work. Not to discourage you at all, just be very clear with the people you're entering into contract (often loose verbal contract) with and make sure you have some contingency plans. Like, if you are reliant on the wwoff site to provide your lodgings, but are reliant on your 'real' job to pay debt obligations it would suck to have to choose between the two!
It can be done, but beware the burnout! If you don't have a partner holding down the wwoof lodging end of things and you realize you just cant hack both you don't want to find yourself in a position of having to sleep in your car for weeks while you double your 'real' job hours just to save up
enough for a down payment. I've seen that happen!