Bummer. I guess someone needs to donate some SSDs to speed up the build times, presuming disk speed is the bottleneck...
...and/or come up with a smart idea like an autodetecting kernel that makes two separate builds superfluous....
...and/or find a way to do it in a single build in parallel, i.e. build everything at once, and then just package up the different components, which should differ only minimally, and output two packages. That would at least mean all the source is only compiled once, the linker would have to run twice on some parts, and the disk image would have to be packaged up twice
Kind of bad when a difference that small requires an entire 6-8h build run

Anyway, thanks for the answer.