gleichzeitig
simultaneously
Happening at the same time or in the same moment.
Man kann nicht gleichzeitig reden und zuhören.
You can't talk and listen at the same time.

simultaneously / at the same time
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simultaneously
Happening at the same time or in the same moment.
Man kann nicht gleichzeitig reden und zuhören.
You can't talk and listen at the same time.

The word is made up of 'gleich' (same/equal) and 'zeitig' (from 'die Zeit', time). So it literally means 'at the same time' or 'with the same time'.
Die beiden Läufer erreichten gleichzeitig das Ziel.
The two runners reached the finish line at the same time.
As an adverb, 'gleichzeitig' (simultaneously / at the same time) is flexible. If it appears at the beginning of the sentence (position 1), the verb follows immediately. But it can also stand in the middle of the sentence after the conjugated verb.
Gleichzeitig begann es zu regnen.
At the same time, it began to rain.
Besides the temporal meaning, 'gleichzeitig' (at the same time) can also connect two qualities or facts. In this case, it means something like 'außerdem' (in addition) or 'zugleich auch' (also at the same time) and does not necessarily describe two actions in the same moment.
Der Film ist spannend und gleichzeitig sehr lehrreich.
The film is exciting and at the same time very educational.
there / with you
'dabei' and 'gleichzeitig' can both show that two things happen at the same time. 'gleichzeitig' says this very directly and neutrally: two actions or events take place at the same time. 'dabei' connects one action more strongly with another action: while you are doing something, something else happens or you do something else as well. Learners may confuse the words because both express closeness in time. If only the shared timing is important, 'gleichzeitig' is more precise. If an action happens on the side or during another action, 'dabei' often sounds more natural.

Meine Freundin hat gleichzeitig zwei Termine und muss einen davon verschieben.
My friend has two appointments at the same time and has to move one of them.
Omar kocht das Abendessen und hört dabei Nachrichten.
Omar is cooking dinner and listening to the news while doing so.