You used a non-dual boot install method so you might be able to use the BIOS to select the boot drive. The keyword is "might." You would test it out. As to not screwing up what you have that's done with full backups of each drive so you can restore if there is an issue.
I would not have used your method as there are plenty of articles like https://www.google.com/search?client=opera&q=how+to+dual+boot+Windows+10+and+7 to get a proper dual boot and install.
I am hoping to set up my computer for dual booting. I have a Dell Optiplex 9020 desktop. It came with Windows 10 Pro on an SSD. It is set up with a "cage" for 2 drives and "ports" for 2 data cords. I just finish adding Windows 7 Pro. At the present moment both drives are installed and operational with UEFI firmware but I do not have both data cords plugged in. I have Win10 in the SATA 0 port and when I want to have Win7 operating I unplug the Win10 data cord and plug the Win7 data cord into the SATA 1 port and Win7 becomes operational. I am afraid to take the next step which is to have both drives data cables plugged in at the same time. It has taken many hours to get things to this stage with quite a few errors since I really didn't know what I was doing. And I don't want to mess things up because I can live with things the way they are now.
So the question is how to do I get this to dual boot without screwing things up?


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