How to make a gap between doors?

Do you make the joint with the same strip that is also stuck to the wall? I was told by a screed layer on MyHammer that you don't do it that way. Seems like there are people on MyHammer who know something...
Quote from the email:

Please do not use edge strips as expansion joints (in the area of doors, etc.). Otherwise, the screed cannot be guaranteed to be plumb from room to room and, as you can see, the joints cannot be created in a straight line.

What else should I use for that?

Why does there have to be a lengthening vowel there? Normally one does not do it that way, but one can do it that way.

Why does there have to be an expansion joint there? The expansion joint is crucial, and that's why you can do it that way. Maybe it will just be a bit crooked afterwards?

Why does there have to be a lengthening vowel there? Normally you don't do it like that, but you can do it that way.

Why ... Why doesn't there have to be one? How should it be, or how must it be? When must there be one and when must there not be one.....

Why ... Why doesn't one have to go in there? How is it supposed to be, or does it have to be? When does one have to go in and when not.....

Comedian!:)

Comedian!:)

But I still haven't gotten an answer whether nothing should go there or something else should go there .....