My 2 cents worth, I agree in a tank as large as a 125 two different clown species could be kept. One at each end and try to put a line of site block such as rock work between them. they will stay, as is mentioned above close to home..I disagree with the idea that you should not mix clowns. You just have to be smart about it.
Its all about space. Keep in mind that Female clownfiash establish a territory, usually around their anemone, and they typically dont stray far, maybe a few feet. I personnaly have a pair of Black and White Ocellaris in a 58 gal with a pair of true percs. There is a little aggression in the center of the tank but one pair has their side and the other has theirs. I made sure that the aquascaping impeded their vision across the full length of the tank. If they can't constantly see one another they settle in better and become more tolerant.
A 6 foot tank, with live rick and coral breaking up the view from one end to the other, provides plenty of realistate for 2 pairs of clowns of either the same species or different.
Another tendancy of fish is to show aggression toward fish of the same color and shape. By selecting to clown pairs that have different shapes and color patterns, you will reduce the chance of deadly clashes.
Don't underestimate clown fish. They can be real tough fish. I had a maroon striped clown that backed a trigger away from his anemone on more then one occasion. I have a tomato at this very moment living with a huge Angler, and a 2 foot zebra moray.well my dad actually wants to get a pair of true percs for his tank. but he has a coral catshark that took down a domino damsel and 3 mummichog minnows with ease, so im pretty sure he'll devour the clowns.