In this example I have a single text frame on all the page. This text frame is made of two columns :
At the red cross (only inside left column), I would like have 2 columns. How to do this ?
You can try the Split Columns feature for that paragraph or paragraphs.
to illustrate what Peter said, a screenshot from my German InDesign 2022 where I use the Split to Columns function of the Paragraph panel with three selected paragraphs:
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Was thinking about this some more while out walking and given the structure there you might actually do better inserting a table.
In any case I think you'll need to do some re-formatting.
I have used this split column window you advise me to use :
The result is good here, but I wonder if it is possible that between the two sub-columns, I could have something like a double line margin (like at the right : see red arrow) ?
I could have something like a double line margin
What do you mean by a “double line margin”? A vertical rule like this?
I was thinking to something like this, between the sub-columns :
You can, with these settings, combining rule above, rule below and paragraph borders.
Of course, you will need to ajust the length of the rules, depending of the column size, it is just a rough.
If one set this to non-printing one could use the paragraph borders as guides.
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"… but I wonder if it is possible that between the two sub-columns, I could have something like a double line margin (like at the right : see red arrow) ?"
That "double line margin" is a non-printing guide.
I do not think that you can get such a guide with split paragraph texts.
If you need a visual hint instead that is non-printing you could use paragraph shading that is set to non-printing:
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