By Washington law you are allowed one deferral for every seven years for a moving violation AND one every seven years for a non-moving violation. If they are both speeding tickets you are out of luck.
You may have another problem. If you are still within the deferral period on the King County ticket you will be found to have committed that one if you are found "guilty" of the Pierce County ticket.
Mitigation will not help you with this since you have to admit that you committed the infraction to mitigate the fines. If you are facing sanctions on two tickets it probably makes sense to consult an attorney to see if there is anything that can be cdone about the second ticket.