| You can do that if you want, but changing the volume of one volume pot will also affect the other pickup, as the pots are in parallel.
Assuming either both pickups on full, or one soloed with the volume on full:
Two 500K pots equals 250K.
Two 250K pots equals 125K
One 250K pot and one 500K pot equals 166.67K
A P with a 250K and a J with a 500K sounds like a good idea, as it will pull back the P pickup's volume slightly, so that the pickup volumes will match a bit better. | 
