Yes they are. However, they will attempt to show their presence when first introduced. But thereafter, they are doing fine.
Once in awhile you can see one try to intimidate the other. Apart from that, they just cannot be bothered with each other. This is what I have observed.
The triggerfishs are definitely not reef safe unless it is species such as crosshatch or Blue Throat Triggerfish. But IMO, if you introduce them into the tank slowly and having a bigger size fish each time you introduce a new triggerfish, you should be able to keep more than one triggerfish. Of course the size of the tank matters as what the rest have said. One of the drawback for having multiple triggerfishs is you should not introduce anymore peacefish fishes such as the fairy wrasses. They will disappear in no time as they will be bullied by the triggerfish(s).
Please consider carefully before getting the triggerfish as (IMO)they are one of the hardest fish one could have in the tank.
I have a 250 gal FOWLR tank and my triggerfishs include: 2 x Blueline Triggerfish; 2 x Humo Humo; 1 x Clown Trigger; 1 x Niger; 1 x goldheart triggerfish. They are living happily with my angels. Enjoy your niger.