mmm sorry my bad english...
i mean if you have, mmm, a complete functional system, with 2 micrcontroller near that you need to have communicated... maybe they are to close (lower than 1m) you don't need any buffer. (but it's suggested)
if you have a system with only one microcontroller. and plan to have some kind of plugable/unplugable extra device (like and hmi or anything that use the other microcontroller) the fact of plug-in and unplug should be taken in consideration.
now for the other stuff.
Rs232 is know for been peer-to-peer comunicacion, just one host and one slave (that in case that you need duplex communication)
if you are planning to have one microcontroller as master (connected to PC) and several slaves (master-slave comunication) i suggest to use some kind of bus...
rs485 seems logical, specially if the slaves are at some distance...
if every thing is very close, i suggest to use/implement I2C (i know that's something ver different to rs232 and usart and so... but i2c is maded just for that, to communicate many devices even in the same board.)
for Rs232 you could investigate about the Daisy-chain implementation...
also to address slaves in RS485 take a look at the modbus specification... for address data sizes, checksums etc....