Hello,
Often capacitors like those have what is called a "Ripple Current" rating. The ripple current rating is a rating that is specified by the manufacturer and is an indication of how much ripple current the capacitor can take before its life shortens a lot.
By paralleling two capacitors, each capacitor gets roughly half the ripple current and so that means you can use a lower rated capacitor if you use two in parallel.
Of course there is a little more to it than that, so that say for example your circuit capacitor(s) had to handle 2 amps of ripple current you probably would not want to use two in parallel that were rated for exactly 1 amp each, as you may want to oversize just a bit and go wth two that are rated for say 1.5 amps each just in case the ripple current in each cap does not divide perfectly.
In power supplies, especially switchers, this is a big issue. Use capacitors that are rated too low and they could actually blow up (spewing lots stuff all about the place). Using two or more in parallel and they usually last a long time when the ratings are chosen correctly. In the circuit that was posted they most likely took all this into consideration, so if you wanted to replace two capacitors with a ripple rating of 1.5 amps and 5uf each, you would probably want to use one 10uf cap that had at least a ripple current rating of 3 amps if one was available.
Of course there is also the ESR issue, where two caps can provide a lower value where one cap may not be able to.
As Brownout had said, if the manufacturer recommends two caps then you had better use two or else know how to calculate the requirements of a single cap that would replace the two.
So the short story is that when two caps are paralleled it's not only the capacitance that is affected but also the ripple current rating
as well as the overall ESR, both of which are VERY big issues in power supplies.