Your plan needs to include checking both models when you buy them to make sure they work correctly.
Then this sparing concept might be a half decent idea. I think a fair number of defects are "DOA" right out of the box or infant mortality (maybe survive a few trips around a loop and then die when something that was questionably manufactured gives out).
There are always exceptions. Something could just spontaneously decide to die by sitting in storage (such as electronic problems relating to the electrolytic capacitors that had a manufacturing problem a while back - not limited to trains). Also, see the thread on the JLC series GG1 side frames that had a casting problem. Many years later (like within the last few weeks! 
) a dealer was successful in creating a spare part, but Lionel had not been able to provide anything for many years. So for that particular case, if you bought 2 locomotives, and they both had the disintegrating trucks due to zinc pest, you potentially could have had 2 non-operable engines instead of just one.
But those are exceptions (hopefully! - though the zinc pest thing does seem to show up every once in a while) for the most part, you'd have a better chance of a functional spare than you would of getting a specific part from the manufacturer years later. There is no such thing as the manufacturer getting a run of "spare parts" these days (except maybe for screws or some minor stock electronic items, which may be able to be bought generically). It's well understood Lionel gets spare parts for the engines by disassembling whole units they purchase with this purpose in mind. So once the engine specific parts are gone, they are gone.
-Dave
