As previously explained the best performance will come from using matched batteries. Also use Deep cycle and not dual purpose batteries as many marine batteries are being passed off by the marketing departments of the battery distributors these days.

If money is a concern and in your case the larger of the two batteries is the one purchased this past spring, it is a deep cycle and not dual purpose and you really have no other use for the battery then I would consider buying a "matching" battery to pair with that battery and run them for the next few years.

Yes there will be a mismatch in the number of charging cycles but if you have taken care of the older battery such as charged it up right after getting home each time and kept it maintained there is no reason that you should not be able to get 3 years or better of satisfactory performance out of the batteries.

When I replaced them then I would put in a matched pair as that is the best.