I have two AT89C5131 microcontrollers.
I would like them to communicate with each other.
I would like to know what kind of bluetooth device can work with this microcontroller and how it would work.
There are many Bluetooth serial adapter boards available online, search e-bay or sites such as Sparkfun. They interface just like a standard serial port and the Bluetooth functions are completely transparent to the micro controller after being paired (which is a simple process)
Well you've got a chip with quite a few communications interfaces...UART (synch/asynch serial), USB, TWI and SPI. Take your pic as to which interface you want to use and find a bluetooth adapter that uses one of these interfaces, read its data sheet to learn how to communicate with it and you're set.