Why there is not possible edit slides and notes at the same time?
Found a post year 2017 : Impress: Edit notes and slides at once? marked as solved. 
Answering the question that you asked last: the poster there, likely, pressed the “solution” button, as the reply that answered their question (“not possible; tracked in the bug tracker”). Does that help?
It helped but does’nt solve the problem itself.
Sure; if it’s not implemented, then no answer on a user-to-user site is going to implement it 
Edited my post. Is it better now?
It seems there’s some misunderstanding here: I think I already answered your question: I said, that it is not implemented; that this site is user-to-user; that you are unlikely to get a better answer than this here; that the request is tracked in the bug tracker (tdf#33603 mentioned in the question that you linked initially). I can only add, that the latter has the “assigned” state, which Sarper set on 2023-11-16.
I really fail to see which other answers you seek here…
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Have you tested with menu Window - New Window?
Version: 7.5.9.2 (X86_64) / LibreOffice Community
Build ID: cdeefe45c17511d326101eed8008ac4092f278a9
CPU threads: 2; OS: Windows 10.0 Build 22621; UI render: Skia/Vulkan; VCL: win
Locale: es-ES (es_ES); UI: es-ES
Calc: CL threaded
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My message should reach to some developer table who take that to programmers who can fix/add this possibility.
Then you need to add yourself to the CC list on Bugzilla, or if there is anything new to add then add a comment.
This is a user to user site.