Windows 10 dual systems - How to make a system selection window during PC startu

Hello everyone.

I've been on the forum for 3 days and I can't find anything that would help me solve this problem. Namely, the situation is as follows:
I had a 250 GB Windows 7 disk (everything works great) and I put a fresh Windows 10 on the second disk (500 GB) and I want to create a bootloader (I think I'm writing correctly), which will contain two systems 7 and 10 to choose from (something as for XP, in boot.ini, add a line with the location of the second system and turn on the system selection window in the 'startup and recovery' window and time for 30 seconds - but since the times of XP it has changed a bit).
I am writing about it to 'create' because the systems were installed separately - during the installation of the 10th, the 7k disk was not connected to the computer (I do not know why?) And all similar topics / guides are based on the installation of the system on a different partition of the disk on which it is already "old system" which gives the machine such a window to choose the system.
I hope that the problem will be solved somehow, because changes in the drive priority bios or their connection / disconnection do not affect the rest of the household - as well as the "new" system.
Regards Adam.

Set the disk with W10 as the first boot device and after booting, enter Win (10) RE (from the W10 command prompt running as administrator : REAgentC / boottore [enter] and restart) and from the WinRE command prompt: bootrec / ScanOs - how he finds it: bootrec / RebuildBcd and add.

If you do it 'the other way around', i.e. add an entry for W10 to the BCD W7 file, it can add successfully but when selecting from the menu seven Windows Boot Manager you can get the W10 entry 0xc0000428 from the Info that 'Windows' was unable to verify the digital signature for its loader.

Or use EasyBCD from this site https://neosmart.net/EasyBCD/

thanks for the answer.
but I think I'm doing something wrong ...
I attach screenshots.

Read slowly and carefully, understanding what I wrote earlier - which means "run as administrator" (google it because repeating this to everyone is BORING - these are the basics of Windows ~ the last decade) and that bootrec is to be run under WinRE, i.e. after reboot ( because it is available there).

For years my account on every computer has been an administrator, I had no idea that there was a difference .. I mentioned that I stopped at win xp and it was a long time ago .. It was enough to write in full sentences, not abbreviations ... But thanks for the answer and commitment, because it's hard to do these days.
In the meantime, I checked the EasyBCD program myself (@krzychupar Thanks a lot) and it went without any problems, in the graphic design of Windows - a miracle, honey, raspberry
Subject to be closed.
Thanks again for everything.

From myself I will add that EasyBCD is a brilliant program, it has saved me more than once where normal methods did not help, for example with the lack of bootmgr and others.