You're going to need a different switch than the standard 3-way blade shown above. It needs to break-connect the bridge humbucker - the white/black connection soldered together above, while also doing the normal switching.
The diagram below will do what you want with a 5-way multipole switch. You'd need to find a similar 3-way switch and use the 1st, 4th, and 5th connection on this one to get what you describe - or almost as it splits both pickups in 4th position.
Here's a 3-way which should work.
You can ask Dimarzio tech support for exactly what you're trying to do and they can probably get you a schematic.