Connect them with SPI bus.
One gets the interrupts and processes those then sends the data instead of by RX/TX to wherever now sends the data through SPI to the other AVR that reads the SPI and sends it by RX/TX to wherever.
SPI (serial) is many times faster than RX/TX serial so the interrupt handler doesn't get held up.
Do you need to send 8 bytes for every interrupt? It seems excessive. If you don't then you could break the 8 bytes down and send a smaller number through between interrupts and not need extra processors.