If I understand your question correctly you want to replace the 500gb hard drive in the new computer with one from a different computer that has W7 installed on it and be able to dual boot to either W10 or W7?
If so then it is highly unlikely that you would be able to get this to work. The W7 installation on the old computer will be "tied" to the hardware on that machine and would be unlikely to work with the hardware on the new machine also there would be problems with the licence activation which is also tied to the hardware. Some info on the subject here
https://www.howtogeek.com/239815/why...ther-computer/
The only realistic way of setting up a dual boot would be to do a clean install of W7 onto the second hard drive in the new machine, to do this you would need a W7 install disk and a valid licence key. There is lots of info on the net on how to set up a dual boot system.