This has been discussed before and a search should bring up some relevant material (look for 5 string J instead of P).
P config makes it easy as you can have identical coils and poles, 3+3 but use demagnetized magnets for one coil in the A string overlap. You can also skip the demag magnets and just use plastic rods for the dummy core since AlNiCo is close to air in terms it's effect as a core.
Just remember you want equal turns*area so the coils want to be the same dimension and the same number of turns. You want the two cores to match as closely as possible too. When it comes to drilling the pole piece holes in your covers you just skip the holes over the dummy poles and that tells everyone which coil is picking up which strings. This is important to know if you later want to add a split J pickup as you'll need to match the sensing configuration to keep all the strings in phase and also hum-canceling.