Hello Mike,
I never replace only one battery in a multiple battery system. As stated by several others, the older battery can shorten the life of the new battery. I learned this the hard way several years ago on heavy equipment, trucks, and farm tractors. When I had a battery go bad, I would replace it and leave the others in place. After awhile it got to the point I was constantly having problems with batteries. I started replacing all of them when one would go bad, and using the good ones in another piece of equipment. Eventually my battery problems were reduced to a level we could live with.
To answer your question, since one is OEM and the other is a year old, I would wait until one of them went bad, and replace both. That is JMHO.
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