Lionel Legacy lash ups

Not surprising.  I've found that some combos do similar things.  I had my two Legacy U-Boats in an MU, and had a similar issue.  I solved it by having the trailing engine flagged as the talking one, that eliminated the dual cab talk.  Since I like the horn to come from both, I also program that way when configuring the MU.

What you are describing is a problem with the earlier Legacy engines.  If you want to run one of these engines in the reverse direction but have the horn, bell, and crew talk only come from the lead engine, you need to designate your train as a number less than 10.  If you build say, train 9, you can have the trailing engines in any configuration you would like and only the lead engine will produce the sounds of horn, bell, and crew talk.  If you run all the engines in the forward direction you can choose any number as the train number.  The newer Legacy engines fixed this peculiarity.