I would have thought that wiring them in parallel is not a good design philosophy since if the internal resistance of one battery increases relative to the other battery, there will be an imbalance in both the discharge and charge rates, which will lead to premature failure of both batteries.
A good point, but in practice it seems not to be a problem.
I can't comment with specific information on the wisdom of connecting SLA's in parallel, but I seem to recall that the concept was approved in theory on some data sheets for SLA's which I have come across. Certainly there are quite a few references to the practice on Google, all of those I saw were favourable, but with some precautions. The fact that your unit was so equipped in a commercial product suggests that the manufacturers were happy with the concept.
I might mention that connecting batteries in parallel seems to be fairly common practice in agricultural machinery and the concept is well accepted and proven there. Most of my equipment used in a former lifetime used 2 x N70ZZs in this mode. We were all cynical when it was first introduced, but no one questions it now.
In your case 12v 7ah SLAs are one of the most popular sizes and are very well priced - probably for this reason. If you were to use two SLAs in parallel, they should be commissioned as two identical new units from the same manufacturer, and I would charge them each as individual units to a full capacity before 'introducing' them in parallel. I would also give them separate fuses to minimize unhappiness in the event of catastrophic failure of one or the other, unlikely though it seems to be.
Having said all of that, there may well be advantages in replacing the entire UPS anyway. You best know the value of your data, and the cost of any interruption to the commercial AC supply, and these need to be given appropriate emphasis in the final calculation.
FORudder