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I really wouldn't recommend trying to do that, for a few reasons. First, in general when you want to dual boot a system, you install the oldest OS first and let the newer OS handle setting up the dual boot configuration. I don't know if Windows 7 will set up a dual boot configuration properly when Windows 10 is already there. It also sounds like you intend to have Windows 7 on a physically separate disk. You might have to do that anyway given that Windows 7 has no native support for NVMe unless you manually apply certain hotfixes that Microsoft says only received limited testing, but that might complicate system recovery situations, especially if you ever need to run Fix Boot Problems, since I'm not sure whether it would set that up properly. I remember someone on here posted a thread where he had spent a great deal of time trying to dual boot Windows and (I think) Ubuntu on separate disks and figured that if he captured an image of both disks, he would always be able to restore either one or even both at the same time and still have his dual boot setup intact. It didn't work out that way on his test restore, and he didn't stick around trying to troubleshoot it. On top of everything else, Windows 7 doesn't support UEFI Secure Boot, which is a nice anti-rootkit measure, so unless you'd be ok toggling that on and off in your BIOS interface every time you want to switch OSes, you'd have to just leave it off, sacrificing some protection that would otherwise be available for your Win10 environment. (That said, if your Win10 disk is currently set up as MBR, it's not using UEFI booting right now and therefore wouldn't be able to use Secure Boot. You might want to look into Microsoft's MBR2GPT utility to perform an in-place conversion to a UEFI setup that would allow you to enable Secure Boot. UEFI has other advantages as well. Obviously capture a Reflect backup before you do that just to be safe.)
Then there's the fact that there are multiple ways to achieve dual boot with multiple Windows OSes when they're on separate disks. The first and default setup is to have the system itself always boot from one particular disk that contains Windows Boot Manager, and then the BCD in that instance of Boot Manager has options for both Windows OSes that would be presented in the blue boot menu you're already seeing. That's generally more convenient, but it means that the OTHER disk might not necessarily be bootable independently if that ever became necessary. The second way would be to set up the disks completely independently of each other, each with its own Windows Boot Manager instance that only knows about the Windows installation on its own disk, and then you access your SYSTEM'S one-time boot menu any time you want to boot from your less frequently used disk containing your less frequently used Windows installation. But I've personally never tried to set something like this up, and Windows Setup wouldn't on its own either. If you want to attempt this, I would recommend physically disconnecting the SSD containing your Win10 installation when you install Windows 7 to the other disk. But once again, if you ever ran into boot issues, this non-default setup might be more difficult to troubleshoot.
And lastly, if your PC is relatively new, you might not even be able to get Windows 7 drivers for all of its hardware. You'd also technically need a separate Windows 7 license, and I don't know of any reputable sources still selling any. And even after all of that, Windows 7 will stop receiving security updates in January, which is a risk unless you'd actually keep that OS completely offline.
If you have games that don't run on Windows 10 but do run on Windows 7 (which actually does surprise me), I would start by seeing if there are workarounds. Sometimes it's literally as simple as enabling Compatibility Mode for that application. If that doesn't work, if they're old enough you might find that they run acceptably in a VM. If you have Win10 Pro, you could use Hyper-V for this, otherwise there's Virtualbox -- although you'd still need a license for the OS within the VM. Otherwise, if this is just for games, I'd suggest buying a cheap used system that already has an OEM Windows 7 license and using that.