Right. Hence the two solutions I provided.
Adjust wiring to split one pickup to the other coil:
For coil split wiring, you can make the connection from the push/pull to ground or to hot. This cancels one coil or the other. Make this swap on one of the pickups and you'll have hum cancelling in the middle position. It makes an impact on tone as well, since a different coil is active in split mode, so you can have some fun experimenting to see what sounds best to you.
The other option here is a benefit of 4-conductor wiring. You can connect the pickup leads differently to reorder the coils. Seymour Duncan has their alternate wiring available on their site, or you can work it out with a multimeter. Why do this? You can leave the push/pull connection to ground, but shift which coil is active when split, which sometimes makes for cleaner wiring.
Bottom line here is if both pickups are same polarity (the typical way humbuckers are) you can have one pickup split to screw and one split to slug (Ibanez and Suhr both do this). Bam, hum cancelling!
This is how to get hum cancelling when you split both pickups to screw or both pickups to slug.
What PRS did with the Custom and all the incarnations of 5-way switching was to make one pickup reverse polarity. So one screw coil is north and the other screw coil is south. They're kept in phase by wiring one pickup in reverse of the other. This way, when you split to both screw or to both slug coils, you get hum cancelling.
You can do this with basically any humbucker set with some careful pickup surgery. Flip the magnet of one pickup and then wire it "backwards". Then refer to the first section above if you need to change which coil is active in split mode.