the Uno just has inputs labelled Tx and Rx also
It does but they are also connected to the USB chip and this usually causes problems.
to connect it to the mega
The Mega has inputs labeled TX1,2,3 and RX1,2,3, just pick a matching pair (say 1) and
Serial1.available() etc etc
to connect it to the mega to use those values on the fly
Don't know what this means. What is "it".
on both chips?
What's the second chip? If you mean the IMU then yes, just connect IMU Tx to the Mega RXn and IMU Rx to Mega Txn.