Adding a 2nd 'Similar' but not Identical Battery?

As batteries age, their ability to charge diminishes. If both of your batteries are only a year in age apart, it would be a lot better than if they are several years apart.

In parallel, two batteries will only charge to the ability of the weakest battery.

What you could do is charge them separately, let them sit, and measure their standing voltage. Compare the two. Possibly use one battery, then when it goes low, use the other one.