Not sure how well this would work without trying it, but I'm thinking your best bet would be to put the Marshall's preamp in the FX loop of the Kemper.
So in theory it would look something like:
Guitar>Kemper input>Kemper FX loop send>Marshall input>Marshall FX send>Kemper FX loop return>Kemper output>Marshall FX loop return.
I don't know how it would sound, but in theory, this would allow you to switch the Marshall's preamp in and out. When the Marshall is in, you would turn the Kemper amp stack off.