I don't see a problem with using two instructors. I did it pre-solo.

Switching between a Cherokee and 172, though, especially pre-solo, doesn't sound good. Post solo, maybe, but it will take you longer in the long run to be equally proficient in both airplanes to a PPL standard. The 172 and the Cherokee land a lot differently and the panel set up is different. Not a big deal post PPL, and even pre-PPL, it would benefit you to see two types. If you are trying to do it in min time at min cost, though, changing planes isn't going to help your goals.

You're probably in a very remote area with few choices, plus, you really limit yourself by only being able to fly weekends.