Don't write off your clowns just yet -- 2 years is still pretty young for the Premnas clowns to be breeding. As Tomoko said, they needs lots of high quality food and very stable conditions.
I am guessing you don't want to raise fish (yet), you just want the relationship?
The only two fish I can think of that will likely pair and spawn and don't compete with another fish you have already are neon gobies and clown gobies. Neon gobies tend to live short lives (2-3 years or so) but are cute and outgoing and will be perched in site often. Or at least one of them will -- I'm not sure which one tends the eggs, but you don't see that one much. If they don't mate, they will coexist peacefully.
Clown gobies will set up house in an acropora and are the easiest to mate since they switch off from male to female and vice versa at any time. You would need a larger acropora since they are obligate coral dwellers. They will strip some flesh off the coral at the base to lay their eggs, but it won't harm a healthy and reasonably sized coral.
Cardinals are another option.
You might also consider a goby/shrimp pair if your goal is more to see relationships than eggs.
Another option might be to acclimate some mollies to saltwater. They will breed... and breed and breed. They are technically a brackish fish, so they can do fine SW although you will usually see them sold acclimated to freshwater. You will need to acclimate them to SW very slowly but no special care from then on except to be sure they have enough algae to eat. You need a male and 3 or 4 females, with only 2 fish the male will breed the female to death. (I have seen SW acclimated mollies for sale, but they are the exception.)
Red-headed gobies are also pretty cool and will spawn in captivity, but I don't know the method of mating them up.
The blue and yellow assessors are much more peaceful than the chalk basslets. The live in the wild in harems of one male to several females, but there's no know way to sex them, so it's rather hit or miss. The yellow assessors are strikingly pretty.