You could try and take a look with 'EasyBCD'to see how your current BCD looks like and this software gives you a good idea how to add your Windows 10 back into the BCD.
If you take a look at your hard disk with Eindows own partition manager ('Disk Management' section of the 'Computer Management' administrative tool), you will see a small partition (100 to 300 MByte) that is usually in front of the main partition(s) on the first hard disk (Disk 0) in your computer. That little partition is where the boot information from both your Windows versions reside. When Windows has started, this little partition is not accessible...as in not showing up in any file manager you care to install in either Windows 7 or 10.
But with '
EasyBCD' you won't have such a hard time fixing things inside this little partition. This change in the Windows boot procedure was introduced in Windows Vista and is activated when you do a clean install on a new hard disk...or when you do a clean install overwriting content of the boot partition of a used hard disk. The creation of this little partition doesn't happen when you, for example, upgraded from XP to Windows 7 and chose to not alter anything in the partition structure on that disk during this upgrade.
The default option, when you do a clean install of Windows 7, it ask you permission to create that little partition. And then you are stuck with it, for as long as you don't re-install.
By adding the SSD the Windows 7 Pro installation detected that the first disk already had that little partition and instead of creating that little partition on the 2nd disk, it uses the one of the 1st disk.The upgrade to Windows 10 detected the BCD settings from Windows 7 Pro and adjusted these.
This implicates that if your Windows 7 Ultimate disk develops any kind of hardware problem, you won't be able to continue working with Windows 10, because that little partition is missing from the Windows 10 boot procedure too. It is likely (but not a given!) that the Windows 10 repair disk will be able to make your Windows 10 disk boot-able. But without such a disk? You will be cursing at everyone and everything during the complete re-installation.
Oh, you think you covered this by creating an image to spool back in case of an emergency? Most image software doesn't get the boot-procedure from a dual-boot system right, especially when that little boot partition does not reside on the disk you are creating the image from, so you will be in the same mess, except with old(er) versions of your files.
Generally speaking, never ever have 2 empty hard disks connected for the creation of a dual boot system. Because then Windows will "decide" which disk gets that little partition where both versions of Windows will boot from. You'd better know the boot order set in your UEFI/BIOS and the numbering of the SATA connectors on your motherboard by heart to have an educated guess which disk will get that little partition. Better connect one disk, successfully install the first version of Windows, then connect the second disk to complete the dual boot setup.