natural hosts will give you the best chance of the clowns however it is possible to get the clowns to enter into a relationship with other species of anemone. It is important to remember however that there are no guarantees as to whether the clownfish will host with the anemone.
In the wild typically A. percula are found in groups with a dominant female a mating male and anywhere from 0-4 non breeders. As you've stated you have a pair of A.percula and A.occelaris that move around in a group, as the two species are almost identical i am not surprised they travel around together although i doubt that there will be a dominant female and a breeder male.
You could get two anemones in the hope that that each pair host with a particular anemone however as i said it is hit or miss.
Another factor to consider is as you've said you have an SPS system, when first introducing an anemone into the aquarium it will move around to find a place that it 'likes' this could be detrimental to any surrounding corals that get stung by the anemone.
HTH
Josh