Marrying pair of 2-year-old-batteries to a similar pair at 75% of their life?

Hello, Wondering if I can marry 2-2year old to 2 similar batteries that have about 75% of life in them.
How does one figure how much life a in batteries?
LOVE to hear from You guyes.......Lotus101

What you mean by marrying?
Do you have a charge controller?
Do the older batteries show a reasonable voltage when fully charged and when part depleted?

A battery life is determined for example by loading it with a known current and measuring how long it can provide current before its voltage drops to 90% of it's rated voltage.
If it can provide a current of 1A for 1 hour then it could be said to have a capacity of 1AH.

The 90% figure is not set in stone, you could say 85% or 95%, but as long as you are using the same benchmark for each battery you test then you have useful data for comparisons of "life"