Nope, you need the Windows Boot Loader to know about other copies of Windows in advance.
If you stick the Win 7 boot drive in the PC, the computer should be booting from whatever disk comes first in the BIOS Setup.
My memory is a bit hazy, now, but you should still be able to switch between the two OS by changing the boot order on the fly
You might still experience problems, though, as the two boot partitions interact.
Most importantly, WIndows 7 will stop being supported, i.e. receiving security updates, at the end of this year. That means about two weeks.
Afterwards, it won’t be a swell idea to plug a Win 7 machine into the Internet anymore.
You have three options
- Bite the bullet and switch to WIndows 10 entirely
- Replace Windows 7 with Linux
- Retain Windows 7 but unplug the network before booting it
Option 3 is not something I recommend in general, but is technically feasible if there is good reason.
Option 1 might be best, if you’re not ready to switch to a different desktop OS.