I can understand the concern about having a back up for the AA Capitole. The problem with your idea is that when the Capitole dies (ie. needs repairs), the whole unit will be down and out, not usually just the transport. Additionally, it is the main board, the DAC chipset (STARS) that seem to be the cause of the biggest problems with these units. Both of these problems would put your unit out of commission.
On another note, the Prima does not really sound anything like the Capitole (I bought a Prima when I sent my Capitole back to France for servicing). The prima's sound stage is a fraction the size of the Capitoles and it can get congested pretty easily with large scale recording (ie. a lot of stuff going on, a lot of instruments, and certainly with rock music).
I do understand your love affair with the Capitole direct to the amps though. Its just too bad its made in France, I have never had any positive experiences with any of the French equipment manufacturers in terms of service and also have found their equipment much, much less reliable. I do like their fries though!