No, not without breaking concentration on Mantle of Majesty
Here is clarification from Xanathar’s Guide to Everything:
As soon as you start casting a spell or using a special ability that requires concentration, your concentration on another effect ends instantly.
Whilst Mantle of Majesty may not be a spell, it is a special ability which requires concentration, thusly casting a spell (or using another ability) that requires concentration causes your concentration to break.
The “(as if you were concentrating on a spell).” is an attempt at clarifying this.
Also, for added clarification from my own comment below this answer:
Mantle of Majesty is described as an ability: “At 6th level, you gain the ability to cloak yourself in a fey magic that makes others want to serve you.” Whilst its description doesn’t use the word special ability, PHB page 45 says this about classes: “Your class gives you a variety of special features, such as a fighter’s mastery of weapons and armor, and a wizard’s spells.” From this we can infer that Mantle of Majesty is a special ability.