Walter the Grump or RetiredKop would be the people to answer your question right. I hope either of them comes on and I'll direct them to your question.
I want to be a part time police officer, but I would like to have a second job as well which would be a part time prison guard would that be allowed?
It may depend on where you live, but you cannot do both here. Frankly it sounds like a bad idea. No good can from having a guard that was the arresting officer.
I'm sure it's 'allowed' dependant upon your employment agreement with either employer about scheduling commitments.