Hi, Michael,
I used the idea of using another plain vanila 8051 uC as the secondary UART in one of my designs. The biggest hurdle at that time, in fact, was getting the two uC's to talk to each other. The intercommunication was done via an 8-bit parallel bus with 4 handshake lines (two pairs of RTS/CTS for each direction to avoide collision). In the end, I felt that much of my time was spent in developing another serious protocol when I already had have two to deal with. Reading your message, I am wondering, if I ever have to do it agian, shall I choose this approach? Love to hear pros and cons or different approaches from you and everyone else here. Thank you.