Connection 2 machine to 1 sim800

Hi if I want connect two microcontroller to one sim800 that 2 controllers can connect by AT command and two micros can share timing that we dont have any conflict to sending at commands to sim800 ,can I use i2c and uart for the microcontrollers or other option without any extend hardware?

You can use two MCU, unless they dont contend for the same UART lines. Make sure that they dont talk to GSM module at the same time. Technically it should work without any problem.