Applying for two part time jobs - need advice

As there are a lot of seasonal jobs around I've decided to apply for a few to increase my chances of getting a job. I've already heard back from two and have interviews coming up.

They are both short hour contracts, so i can work at both but what if they want me to work the same shifts. How do I go about this? Do I tell them about the other job or will that affect my chances of getting hired?

You could give them both different availabilities, so that they dont clash.

On my CV they asked for availability and I put anytime as I was only expecting to have one job, so would I be able to tell them my availability has changed or would that look bad?
I've never had a part time/temporary job before, I've only ever worked full time so I don't really know how this works.

In that case, id let them know that you may be taking on 2 jobs. That explains your availability change, and doesnt sound too bad (there are worse reasons for suddenly changing your availability). And that you would rather tell them now before you start so that they can give you shifts you can do (and dont clash) to start with rather than you having problems later and letting 1 job or the other down.