It's not the easiest combination of clowns to mix. Generally speaking the "rule" is not to mix clownfish. If you're not hell bent on the second pair being black you would probably have a much better chance of adding Clarkii clowns.
I've got this combo in my tank right now and have never seen any problem at all. They actually hang out together a lot of times and even stranger they will swim off in different pairs which amazes me every time I see them do this. It's like the ocellaris doesn't know the other one is a clarkii.
Mine however are all pretty small (1.25 to 1.5 inches) with the clarkies being slightly bigger then the ocellaris. Also worth noting, they say that clown fish get more aggressive with age so even though they get along fine now in my tank, doesn't mean they will in the future but I'm hoping they will be used to each other and "stay friends".
My tank is 120 and yours is bigger then mine so they will have space. I wouldn't try it in anything less then a 90 but that's only my oppinion.