If you have a pass from WDW with days remaining on it, you can take it to Disneyland and have the deduct 1 day off of your pass and give you a 1 day DL pass.
If you want to go a second day, you would do the same thing the next day.
For as many days as you wish. They will only do 1 day at a time.
You cannot take a DL pass with days remaining and do the same at WDW.