TRIS change?

Hello.
I have a simple question: can I use one port as an input and as output?
I mean, I want to send 1 bit data from MCU A to B with one wire, so one idea came to my head: what if I just send a bit with wire, than change the according TRIS register and wait for response? Can I change TRIS in the middle of the program many times?

Yes, of course you can, and it's a VERY common technique - most commonly used to simulate an open-collector output pin, such as for I2C.