Unlike with most fish, my suggestion would be to add them at the same time. This is what I did. My theory behind this was that they're both bold temperments and if I let either establish territory first, they'd surely beat on the other once it was introduced. I think that by adding both, I may have actually cut down on the aggression. This is one of the few circumstances where I'd say to add two fish at the same time. Just keep an eye on water quality.
Also, add the fish with just the moonlights on the tank, no daylight/white light and don't feed 'til the next day.