The problem with two instructors is that they will teach slightly different recipes. You have enough on your plate without having to get confused with two different methods. You will take more hours to complete your PPL in my opinion.
When my instructor has been on vacation I have used a couple of different instructors, and at one point I was wondering if this was wise. in retrospect I should've probably refrained and waited for my instructor to return as I don't think I gained a great deal other than some more hours in my logbook and learning that you can turn off the carb heat either on final or on the ground. Having the option though means that I tend to forget it at all if I'm not careful.
Cheers, Howard