Here is my two cents, for what it's worth:
I am generally of the mindset that you should work hard and try to treat every job as though you will work there until you retire, even if you know full well that it is only a temporary stepping stone. That being said, these are both clearly temporary and have nothing to do whatsoever with what you will ultimately be doing for a living. So even though earning as much money as possible for the next few months is important, the bottom line is that in the grand scheme of things, if you lost either or both of these jobs, it would not be the end of the world for you. It might kind of screw up your short-term plans, but it is ultimately not going to have a huge impact on your life.
With that in mind, do the math and figure out which job stands to earn you more money, given both the pay and the likely number of hours. The one that stands to pay you the most is your primary. The other is your secondary.
Go to boss at primary job and say something along the lines of: "I have another job temporarily because I need to earn a bit more money. This job is more important to me, and you have my commitment. But I need somewhat of a commitment on hours. Can you give me a rough idea of the max hours you can schedule me? If you can maximize my hours here and give me at least X hours, I will commit to working those hours and will make sure the other job doesn't schedule me to conflict with the hours you are giving me. If they cannot accommodate that, I will leave that other job. But that makes it all the more important that I have a commitment about the minimum number of hours from you."
Go to boss at secondary job and say something along the lines of: "As I think you know, I am in a situation where I have to work two jobs to get by for now. I am doing my best to juggle both schedules, and it would help me tremendously if either my schedule here were more consistent or regular, or if I had a better sense of how many hours per week I can expect to work. Can you please help me with this?"
See what the feedback is. One may be more willing to commit to you than the other. If that is the case, then despite a difference in the number of hours worked, that factor may change which is your primary and which is your secondary. But I think you have to have a heirarchy set in your mind so that if you are forced to choose at some point in time where there is a conflict, you know which way you have to go.