To have both I have heard of some creating a system image of 7, splitting the drive into 2 partitions, upgrading to 10, then installing the image of 7 to the newly created partition on the same drive then at boot selecting 7 or 10.
Windows 10 when downgrading goes back to the original 7 install as it creates a backup of 7 before the upgrade so you can go back to how it was. After 30 days the option disappears.
Other less complex and easiest method is to either get a cheap computer that has 7 on it and use that for 7 and upgrade current system to 10, or buy a new computer with 10 on it and leave old system at 7.