Hello. Im trying to create wireless car but im stuck in one uncool problem - two supplys and two MCU (Attiny2313 and Arduino Uno(atmega328)) I have two supplys and two different voltage and in this case devices cant communicate to each other. To communicate i need something who looks on voltage and control it.
Maybe someone know how to deal with this problem. 
A scematic would help. Why do you have to use 2 supplies?
Sounds like you may need a voltage translation chip like the TXB108. Adarfuit sells a breakout board for $8. You can use this board for SPI, UART, etc. but not for I2C communications. For I2C, you need a different type of board like this one from Adafruit for $4.