The recommended order to install is older OS first,
1.- Or disconnect the Newer OS system disk before make your install on the second disk an older OS, remembering to first disable secure boot and enable CSM or CSM + UEFI on the Bios, usually there is a fast Key (F7, F8, F12, etc) to let you select from what disk you want to boot,
2.- To boot Win 7 to 10 from a MBR disk it needs usually an active (System Reserved) NTFS partition with the bootmanager and Boot folder/files where the BCD (for MBR) is, you can use also a Fat-32 active partition with mentioned files and folder.
3.- If you use a Fat-32 active (as System Reserved) partition on a MBR disk it is capable to boot a MBR install + also capable to boot an UEFI system.
If you have a System Reserved partition on your MBR disk you could format it as Fat-32 and make it active, and run the command suggested on No. 4 and maybe adding all required to boot the 10 UEFI, as the EFI folder and editing the BCD (for UEFI) located into EFI\Microsoft\Boot after copy it and all needed from Win10 disk to this location. Don't forget to also open your Bios and select this disk as the first option to boot, disable secure boot and enable CSM or CSM + UEFI on the Bios, usually there is a fast Key (F7, F8, F12, etc) to let you select from what disk you want to boot, if you want to boot 10 directly.
If that don't work you need to follow what said on No. 1.
4.- Useful info to create all boot files/folders:
Boot from a WimPE, format the System Reserved partition with the required format and run in a command promt (on each disk having the other disconnected) to create the boot files/folders:
(Recomment to read carefully the info on the link first: https://docs.microso...ons-techref-di)
bcdboot C:\Windows /s S: /f ALL
C:\Windows = Where Win is installed (just change S: for the letter where Windows is located as seen on the WinPE)
/s S:: = Where the boot files/folders will be installed (just change S: for the letter of your System Reserved partition as seen on the WinPE)
/f ALL = Installs MBR + UEFI boot files/folders (can be omited if not required and only those that apply will be installed automaticaly)