Sim

George R. R. Martin comentou sobre isso mesmo que ele só explora os mentores atuais.

Maester Aemon is doubly sworn, to both Citadel and Night's Watch. That is true of the maesters at Eastwatch and the Shadow Tower as well.
_{So Spake Martin, SF, TARGARYENS, VALYRIA, SANSA, MARTELLS, AND MORE}

Eu perguntei a Elio e Linda essa pergunta no Twitter e, embora eles não consigam confirmar, acreditam que foi o caso historicamente.

@westerosorg Hello, just a quick question. I know that the 3 current maesters of the Night's Watch are sworn to both orders but was this the case historically too? Thanks.
I can't say for sure, but I would be surprised if it was not the case that that was normal procedure.
_{Twitter, westerosorg}

Isso também é explicado nos livros.

That did not sound right to Jon, yet he said nothing. The maester was a hundred years old, and a high officer of the Night's Watch; it was not his place to contradict him.
_{A Game of Thrones, Jon VIII}

A toothless smile quivered on the ancient lips. "Only a maester of the Citadel, bound in service to Castle Black and the Night's Watch. In my order, we put aside our house names when we take our vows and don the collar." The old man touched the maester's chain that hung loosely around his thin, fleshless neck. "My father was Maekar, the First of his Name, and my brother Aegon reigned after him in my stead. My grandfather named me for Prince Aemon the Dragonknight, who was his uncle, or his father, depending on which tale you believe. Aemon, he called me …"
_ibid

Sam donned his blacks to say the words, though the afternoon was warm and muggy, with nary a breath of wind. "He was a good man," he began . . . but as soon as he had said the words he knew that they were wrong. "No. He was a great man. A maester of the Citadel, chained and sworn, and Sworn Brother of the Night's Watch, ever faithful. When he was born they named him for a hero who had died too young, but though he lived a long long time, his own life was no less heroic. No man was wiser, or gentler, or kinder. At the Wall, a dozen lords commander came and went during his years of service, but he was always there to counsel them. He counseled kings as well. He could have been a king himself, but when they offered him the crown he told them they should give it to his younger brother. How many men would do that?" Sam felt the tears welling in his eyes, and knew he could not go on much longer. "He was the blood of the dragon, but now his fire has gone out. He was Aemon Targaryen. And now his watch is ended."
_{A Feast for Crows, Samwell IV}

Embora isso só fale sobre os atuais maestros, o wiki considera que também é o caso historicamente.

Maester's serving in the Night's Watch on the Wall are doubly sworn, and uphold both the maester's vows and the oaths of the Night's Watch.
_{A Wiki of Ice and Fire, Maesters}

Esta informação pode ter vindo de O Mundo do Gelo e do Fogo, que indica que os mestres históricos da Guarda pertenciam a ambas as ordens.

The tales of long-dead rangers written down by maesters of the Watch all agree that the giants did not make homes or garments, and knew of no better tools or weapons than branches pulled from trees.
_{The World of Ice and Fire, Ancient History: The Dawn Age}

Note que os únicos três maestros que conhecemos para a Patrulha da Noite são Aemon (Castelo Negro) , Mullin (Torre das Sombras) e Harmune (Eastwatch) todos os quais são representados como membros da Watch. O único outro exemplo é Samwell Tarly, que é novamente um membro da Watch, embora ele seja um membro antes de se tornar um meistre. Não está claro se os outros eram monges ou irmãos primeiro.

Sim, parece que tanto o Eastwatch quanto a Torre das Sombras possuem Maesters.

O GRRM confirma isso em uma entrevista:

Maester Aemon is doubly sworn, to both Citadel and Night's Watch. That is true of the maesters at Eastwatch and the Shadow Tower as well.

_{So Spake Martin, Entry 2999}

Quanto à forma como os Maesters vêm para castelos, se eles já são Black Brothers (como Sam), peça para ser atribuído lá (como Aemon), ou apenas uma terrível sorte, uma vez que eles forjam sua cadeia não é totalmente conhecida.

No entanto, parece que a Patrulha da Noite pode pedir à Cidadela por um Meistre

"You can. You will. Maester Aemon is old and blind. His strength is leaving him. Who will take his place when he dies? Maester Mullin at the Shadow Tower is more fighter than scholar, and Maester Harmune of Eastwatch is drunk more than he's sober."

"If you ask the Citadel for more maesters . . ."

_{A Feast for Crows - Samwell I}

Mas também implica que o Maester possa ser o Night's Watch Brothers primeiro; Não muitos outros Maesters que vemos podem ser descritos como "lutadores" ou "bêbados" ...

Limpar como lama!