Rule #1:
Never believe the person at the fish store. They are selling stuff to you.
Even the good ones who aren't out to make a quick buck are often years behind the times on what does and doesn't work. Always do your own homework and find out if something will work from the experts.
There are exceptions (like when you have to take in a friend's fish due to some emergency, etc.) but generally doing your own research and finding out what the experts say will serve you better than believing the people at the fish store, who do have a vested interest in selling whatever they can.
Now, I agree that adding the tangs at the same time will decrease violence in the short run, but as they get bigger and want more territory, those Zebrasomas are going to look at each other as territory invaders. Fish seem, more often than not, to be less violent and territorial the younger they are. As they get older, these traits come out. The best comparison that comes to mind is keeping a kitten and a hamster together. The kitten doesn't look at the hamster and think, "Kill!" The cat the kitten turns into is much more likely to... Sometimes not, but often enough to be advised against unless you really know what you're watching for.
Rule #2:
Don't always trust answers on a forum.
Posters believe certain things, sometimes from experience, sometimes not. However, they have no money to gain from misleading you into more expensive options. They may not always give the simplest answer due to issues of personal pride, predjudice, total lack of knowledge, etc. Your best bet is to actually double check whatever you are told if it's important.
Robert Fenner (probably the expert I trust most so far) is of the opinion that Zebrasomas can be kept together, but must be watched VERY closely. He also says 50 gallons is the dead minimum for the genus. However, that all comes after this quote:
"Sailfins are better to keep just one individual of one species to a system. They can and will fight with their species and other tangs, seriously damaging other similar appearing fishes, even in tanks of hundreds of gallons." He also comments that they will allow the other Zebrasoma to retreat in a tank of sufficient size. 65 gallons isn't enough to let that really happen. You assume one wins and takes up maybe 50 gallons as territory and the other is left with the other 15.
Further checking shows that the Purple Tang is generally peaceful while under 3 inches in length, but not so afterwards.
http://www.wetwebmedia.com/zebrasom.htm
http://www.wetwebmedia.com/YellowTang.htm
http://www.wetwebmedia.com/z_xanthurum.htm
http://www.wetwebmedia.com/paracant.htm
Make your own call, but people aren't warning you as an attack on you, but based on a lot of people having problems with scenarios somewhat like the one you describe.