Combining two preamp pedals in one enclosure w/ additional features?

[slowpogo]

I recently made two Aion preamp pedals, the EP3 and Korg SDD-3000.  I'm very happy with them and they're basically "always on" pedals for me.

I've made a lot of projects but I'm a n00b with electronics design. Broadly speaking, what are the challenges involved with combining two such preamps into the same enclosure, but sharing a single power supply?  It would also be great to choose between serial and parallel operation, with a blend knob, and to be able to choose the order in serial mode.

How much of this could be done with simple wiring and pots/switches, and how much would require more complicated circuit mod/design?

EDIT: I should mention they both run 9V center negative.  Not that I'm asking anyone to get super specific but here are the build documents in case someone wants them.
Echoplex EP-3: https://www.dropbox.com/s/h5ogujjyxzbwj02/aion-ares-echoplex-preamp-documentation.pdf?dl=1
Korg SDD-3000: https://www.dropbox.com/s/5x723brxdo57va6/aion-eclipse-sdd3000-documentation.pdf?dl=1