This has been resolved - instead of merging the channels and removing channels 4 and 5, I've replaced the data within channels 4 and 5 with the calculated data of the merged channel.
So I still have the 6 channels but channels 4 and 5 no longer show left and right channel data but instead show the merged channel data.
I attach the custom process that I made here.
process_weighted_merging.m (5.5 KB)