Yet another alternative is to use a small microcontroller with three serial ports (UART's). Send commands to it from the R-PI with information to which of the two other serial ports to forward the data to, and receive packages of received data from the two other serial ports, with information pertaining to from which of the two serial ports the data was received from.
Obviously you then also have to get/write the scheduler software for the microcontroller, but I have done something similar in the past. the advantage is that the main CPU doesn't have to have SPI, only a serial port.
The parallax propeller http://www.parallax.com/tabid/.....ult.aspx
is an ideal little chip for such an application, its possible to program it to have eight UARTs.