Just to be clear and I'm sure you're aware of that, running Windows 7 and Windows 10 in dual boot with the same key is illegal.
Since the latest Windows 10's November update (1909), Microsoft has changed its Win 10 installer to accept Windows 7 and 8, 8.1 keys. Because of this change you are allowed to do a clean boot of Windows 10 2019 November update and use your Win 7, 8 or 8.1 keys.
After using the Windows 7 key, Windows 10 will report this key to Microsoft’s servers, and you'll receive a digital entitlement or digital license to continue using Windows 10 for free.
With all this said, if you create a dual boot with Window 10 1909, you'll be able to use and activate it with Windows 7 key and I seriously doubt that your Windows 7 will then be deactivated, but I just can't confirm this since I haven't tried it.