Add a second battery?

Last August I bought a new lead acid battery for the trailer and all has been well so far. We will be doing some boondocking this year and I want to add a second battery for a little better run time on my lights, water pump, run the generator less time, etc. I know one should buy batteries in pairs, however, I hate to buy 2 batteries with one being so new. If the only negative is that the pair may not last as long since they were not paired at purchase are there any other potential problems I might face? While I hate buying 2 new batteries now I would really hate buying 1 now and then 6 months down the line I need 2 more. Any info will be greatly appreciated.
Thanks,
RichH