Efficient Communication between two older 8051's

Here's the situation.

I have a system that needs to interface with an HM-TRP radio module and a computer. Both devices are set to run at 9600 bps.

I initially tried hooking up both devices to a single AT89S52 chip and tried software UART, but I don't think the system is up to speed (even with the 22.1184Mhz crystal) as other things needed to be processed in the micro.

Now I decided I need two micros. I will use the AT89S52 for most of my processing and hook its hardware UART pins to the computer through a converter (think max232 IC), and I will use the AT89C4051 for the radio module processing and hook its UART pins to the radio module UART pins.

After working on the main board with AT89S52, what I have available to the UART are 7 data lines and a clock where the output changes every clock cycle.

This means I won't be able to send whole bytes between the two devices right away, but what I want to do is make AT89S52 do all the functionality and use AT89C4051 as the radio module translator so I can quickly read from and write to the radio module.

I heard about SPI, CAN, i2c bus communication modes, but which is best for me and why?
Is there even a better communication mode designed specifically for the 8051 that I don't know about?

Please advise.