Dave, thanks for your suggestion about tethering the remote. Tried this but no results.
I did try using Fixed channel 2 on the test track and could then add the two engines. (I have the Rev L TIU.) So apparently Channel 1 is the culprit. After I added the two engines, I could then run them via Channel 1. Many thanks for the suggestion to try a different channel. I haven't had time to look at the thread but will do so later tonight. Meanwhile it's a relief to have two engines back on track!