Separating the Tx and Rx lines on 2 meters in the IC746 could not be easier.
Put the rig on the bench with the front panel facing towards you, and turn upside down, with the front panel still facing towards you.
Remove the bottom cover.
At back-left there is a 5 inch square metal screening plate held in place by four screws - remove it.
You will see a miniature co-ax plug and cocked, labeled VRX. Simply unplug, tuck the original plug out of the way and plug in a new plug attached to a length of miniature coax, which can neatly leave the rig through the hole on the back panel that contains an earth bolt. Now you have your separate receive input!
You could cut off and re-use the miniature co-ax plug, but I preferred to contact my local Icom dealer and buy a new one (a couple of dollars) - the Icom description is: PLUG TMP-P01X-A1 (Min Coax) IC-125.