I would put the two new tires in the back, then the old ones in the front (in hopes that you're replacing the old tires really soon). I wouldn't put two different tires on the same axle, even if they're the same size because manufacturers sometimes have different specs and this results in different size tires. I have two sets of 15's with different brand tires, but the same size. The height of one tire differs from the other, so I'm guessing that it might be the same for you as well.