I installed tow-boot on SPI, and that got around the problem - in effect tow-boot serves as the alternate session, I guess. But when booted from tow-boot, neither OS can see my USB wifi, I suppose tow-boot was too interested in it as a boot device. So until I sort that out, I'm back to popping the SD every time.
Not that tow-boot worked very reliably anyway.
Not that tow-boot worked very reliably anyway.