I looked at images for recumbent tandems. Turns out someone made a tandem like what you are proposing. Search for cruzbike tandem. I'm not really sure it helps you, the back half is huge.

Seems to me that you can easily remove the timing chain and split a recumbent tandem without much issue. S&S makes some pretty big couplers, but if you really need a bigger keel beam, you can go with two tubes.

On edit: I asked someone that knows a lot more about 'bents than I do, and it turns out I should have looked on the S&S site. This one would work pretty well with a cruzbike front end. You could split it in front of the rear crank and behind the captain's seat. Barcroft Columbia Tandem Recumbent with S and S Couplings