If two pics share RW,E and RS line, then E pin of master pic(master is connected to DATA LCD pins), enables slave with E=0 and disables LCD. So pic could talk with each other using synchronous comm, or async. Eg I2C, or like, half of SPI etc, or Tx and Rx for async. When master want to free comm, just send dome break signal, or on transition from 0 to 1 on E pin slave pic just relist RW and RS pins by setting them to inputs.
And I don't see why is I2C problem?
It simple, robust protocol that can be done on that 3 pins. Most LCD have pull up on all pin, so it is very well suited for that task.