Dave,
I still can not find this directive in your DCS O gauge Companion book
In the 3rd edition, it's on pages 53-54, as follows:
If a DCS engine is added to more than one remote, it will not necessarily be assigned the same DCS ID# in all remotes, but rather, a DCS engine will have the DCS ID# assigned to it by the last remote to which it was added. This is the DCS ID# by which the engine must be addressed. Any remote that has the engine with an ID# different from the one in the engine's memory will be unable to command the engine and will get errors when it attempts to do so. Therefore, it is always advisable to keep DCS Remotes in sync by ensuring that DCS engines have the same DCS ID# in all DCS Remotes.
If engines are to be added to more than one remote, it's important that they always have the same engine ID# in each remote. When adding an engine, first add it to one remote and then repeat the ADD ENGINE command for each additional remote. If you follow this process starting with the first engine you add to DCS and continue it with all subsequent engines, your remotes will always be in sync with regard to engine ID#s.
When changing an engine ID# of an engine already in one or more remotes, ensure that the engine ID# to which you desire to change the engine is available in all remotes where the engine resides. To keep DCS engines in sync when changing their DCS ID#s in multiple DCS Remotes, first change the engine ID# in the first remote. Once the engine ID# has been changed in the first remote, delete the engine from the second remote and re-add it to the second DCS Remote. Repeat these steps for each additional remote in which the engine's DCS ID# is to be changed.
For additional technical explanation of DCS engine ID#s, refer to Appendix E: The Truth About DCS Engine ID Numbers.
As noted, Appendix E, on page 229, has a more detailed, "under the hood" discussion of DCS ID#'s.
This and a whole lot more is all in MTH’s “The DCS Companion 3rd Edition", available for purchase from many fine OGR advertisers and forum sponsors, or as an eBook or a printed book at MTH's web store!