dabac

I've strapped the wheels (vertically), one on each side to the rear rack, then strapped the frame incl fork also pointing upright to a 45 L backpack.
Worked OK once I stopped the bars from swivelling, and rearranged the contents of the pack to get the frame further from my head.
Carried like that for 16 miles.
If I were to make a habit out of it I'd probably make some sort of tow bar.
A fairly simple option would be to make some oversized fender struts (and something to secure them vertically) and use them to anchor a q/r hub basically where a fender mounted rear light would be. The front wheel of the towed bike could be tied to the frame of the towed bike.