Double headed locomotives require speed matching to run well together, especially true for switching.  I used page 50 in the NCE PowerCab manual rev.1.65 .   Control Variables ( CV''s ) should be changed in programming mode so that the initial setting of a CV ( Cryptic Vexation? ) can be recorded.  I chose to not modify the CV's of a Broadway Limited 2-8-0 steam locomotive, matching the CV's of a Bowser diesel to it.  I later repeated the process with two Bachmann Sound Value Alco model diesels.   I was slightly surprised that out of the box the FA2 ran much faster than the RS3.  Perhaps prototypical, but undesirable.  Most surprising was that the CV's mostly read 0 !  With trial and error the two were adjusted to match the 2-8-0.  There are more sophisticated ways to speed match, this has been adequate for my purposes. I think Joe Fugate's next "Run Like a Dream" locomotive book will include sophisticated alternatives.  

Google Spreadsheets are a good resource with some fancy functionality, in this case I used only the grid to keep track of CV's.  Should CV's go awry and things not work, there's always lucky option 7 for recovery programming, seen on page 54 of the NCE manual.  Remember, option 7 also resets the loco id to 3 !

https://ncedcc.zendesk.com/hc/en-us/articles/204241665-Locomotive-Speed-Matching-for-consisting

https://www.bachmanntrains.com/home-usa/board/index.php?topic=28316.0;wap2