At the "Master"

Have the master serial port's receive ISR put its data into a ring buffer, in the normal way;

Then The I2C pulls bytes from that ring buffer, and transmits them

At the "Slave"

The I2C receives bytes, and puts them into a ring buffer;

The slave's serial port transmit ISR pulls its data into from a ring buffer, in the normal way...


Interrupt-driven, ring-buffered, serial IO:
See: http://www.keil.com/download/docs/200.asp
And: http://www.keil.com/download/docs/71.asp