Generally, assume the lower result unless explicitly stated otherwise. If there are two pins supplying 12vdc but only a single output rating, then the assumption should be that the PSU produces a single 12vdc rail, and the total of both pins is 5A max. It is implied (unless otherwise stated) that the full rating of 5A can be drawn from just a single pin.

From a marketing perspective, if there were multiple output rails, they have an incentive for them to list them out in detail. ATX PSUs for PCs do this.

From a safety perspective, it would be downright irresponsible to design a connector on a finished product (like this standalone PSU) that has a lower per-pin rating that what the supply can offer, so any decent pre-built PSU will not have per-pin limits that are lower than the total output limit of that group of pins. As a counterexample, ATX PSUs are a component in a larger product (a computer) and so individual pin limits must be adhered to.