I would not attempt it in a tank your size... if you're talking about your 90gal.
While a 90gal would be considered borderline for mixing angels, the flame you have is already established, and it could be too stressful for both fish.
It would be better to introduce the two angels as juveniles at the same time.
It would also be better to have a lot of rockwork to separate sections of the tank so they're not staring each other down all the time.
With that being said, I'm going to be attempting to mix a couple of dwarf angels in a 125gal soon.
It already has a coral beauty in it, and I plan on adding a flame angel, but only after I add some more rock and rearrange the tank to breakup any existing territories.
When I do finally get around to it I'll post how it goes.
There's a great article on Majestic Angels over at AA: http://www.advancedaquarist.com/2005/7/fish
While no angel is 100% reef safe, according to that article the Majestic is one of the more reef safe angels.
WWM has a huge FAQ regarding angelfish compatibility.
Check it out here: http://www.wetwebmedia.com/marine/fi...elcompafaq.htm
Keep in mind that even if you read a couple of success stories from people with tanks your size or even smaller, those cases are the rare exception to the rule, and for each "success" there's probably twice as many cases where the angels end up fighting until one is removed or killed.