multiple cookie irule

We have an iRule set up that sends all traffic with a .dll extension to a specific pool. This pool has the same number of members as our default pool which has all the .php traffic going to it (also part of the iRule). Our thoughts were that we needed the same amount of members in each pool to retain server affinity. Say you have a php session open and then decide to jump to a page that uses the .dll we want to make sure that you do not lose your .php web session or vice versa. My question then becomes, can we also have the irule use a different cookie for server affinity for the traffic routed to the pool for the .dll traffic so that a subsequent request for a non .dll page would retain its original server affinity?