You can not mismatch your tires. As per the PCA.org tech site here is what the Boxster expert Scott Slauson says in answer to a members question:

Q: In looking at the diameters for these tires, they are 23.4 inches and 25.1 inches. Is this going to be a problem? I am planning to use these tires for Drivers Ed events.

Ans: "The one thing to consider though is when changing tire sizes to keep the rolling circumference even on both front and rear tires. The ABS system will shut down if the circumferences are too large of a difference. Those sizes will not work with the ABS system on the car. You will get a speed deviation that will shut down the ABS and turn on the light."