I bought a hybrid '59/Custom5 from JumpMarine last week and it only has two leads. Can the leads joining the two coils be unsoldered and extended to split the coils?
Nope, not without major surgery. Are you sure you need a split? You can buy a '59 and other pickups that normally come as a 2-wire as a 4-wire from the vendor, such as Musician's Friend....
yes, in a certain way you can create a similar sound with a little device called Q-Filter, which is sold by Bill Lawrence. This dummy coil can be dialed or flipped in to sound much like parallel single coils. I used it with a JB until i switched the volume pot to a push pull. Cheap, easy to install, the sound is great.