One add-on comment:

A serial loop... what's wrong with peer to peer (assuming you have a tasker)?

I agree to just make a TXD/RXD loop between the processors. But you must consider the trade-offs if you are using FLASH chips that require the TXD/RXD to download programs or if you need to support an external serial link. I'd recommend adding a cheap mux controlled by one of the uC; when the uC resets and takes its outputs high, they need to flip the serial link into the configuration for FLASH download (if used). Since part of the FLASH download process is to reset the micro, the mux will be affected by the reset giving you access to one of the micros. Normal initialization can flip the mux to CHAT MODE.

To download FLASH the other micro, you can work out the additional protocols. There are many ways and I'd rather wait to see if it applies and to hear more specifics.

aka j