
zugleich
simultaneously
At exactly the same time; at once.
Die beiden Läufer überquerten die Ziellinie zugleich.
The two runners crossed the finish line simultaneously.
simultaneously / also
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simultaneously
At exactly the same time; at once.
Die beiden Läufer überquerten die Ziellinie zugleich.
The two runners crossed the finish line simultaneously.

also
In addition, furthermore; expresses that something has another quality or function.
Dieses Smartphone ist elegant und zugleich sehr leistungsstark.
This smartphone is elegant and also very powerful.
The word is made up of 'zu' and 'gleich'. It expresses that two things come together ('zu') at the same moment or in the same state ('gleich').
Er ist mein Kollege und zugleich mein bester Freund.
He is my colleague and also my best friend.
As an adverb, 'zugleich' (simultaneously/also) can stand at the beginning of a sentence and then causes inversion (verb in position 2). But it can also stand in the middle of the sentence, usually after the conjugated verb.
Zugleich wurde ihm klar, dass er einen Fehler gemacht hatte.
At the same time, he realized that he had made a mistake.
The meaning depends on the context. If 'zugleich' (simultaneously) refers to two actions, it means 'at the same time'. If it connects two qualities or roles of a person or thing, it means 'in addition' or 'also'.
Er kam an und ging zugleich wieder.
He arrived and left again at the same time.
also / besides
Außerdem adds another piece of information without emphasizing a particular connection. Zugleich can also name another quality or function, but it often emphasizes that both exist together in the same person or thing. Zugleich is therefore more suitable for a dual function.

Nora ist Leiterin des Projekts und zugleich Ansprechpartnerin für die neuen Mitarbeiter.
Nora is the project manager and, at the same time, the contact person for the new employees.
Nora leitet das Projekt und betreut außerdem die neuen Mitarbeiter.
Nora is leading the project and is also supervising the new employees.