*EDIT*: I accidently typed 'Into' as the first word of my topic title rather than 'Install', but it's too late to correct that now. I'm sure my meaning will get across.
I recognize my topic title is a bit confusing, but I couldn't think of a better way to word it, explanation:
I would like to install Windows 10 and Windows 7's boot files into the same partition, without merging 7's boot entry into 10's loader (i.e. I want them to have independent loaders, BCDs, etc, without having to chainload 7 from 10's boot menu). As of right now, I can install 10 and 7 with imagex, use the 'bcdboot' command to install 10's boot files, then install the Windows MBR data with the 'bootrec' command, followed by running 'bootrec /rebuildbcd', which lets me add an entry for 7 into 10's loader. But I don't like this, because when selecting 7 from the menu, the PC reboots, and I have to type in my VeraCrypt encryption password *AGAIN*. I don't see why 10 cant just load 7 from its' menu without a reboot.
So, I figure there must be a way to maintain independent boot files and BCDs for each OS, so that's 10's boot loader will only boot 10 and have no entry for 7, and vice versa for 7's loader. The plan is to encrypt, type in password on upon powering on, then choose *EITHER* 10 or 7 from the GRUB2 menu, in which case it should go straight into loading the selected OS without a reboot or OS choice prompt. As it stands now, GRUB2 only has an entry for 10, because 10 and 7 use the same loader and that is all GRUB2 sees. There are no other issues with loading 7 from 10, and 10's loader is superior when it comes to UEFI (but not so much for legacy), but I would still like to keep their loaders separate.
The issue is in installing the boot files into the same primary partition while keeping them separate. I'm using the following command syntax:
bcdboot C:\Windows /l en-US /s A: /f BIOS
I have to designate C to the desired Windows OS (depending on which I want to create boot files for), and A: for the destination boot partition. I would just store each OS's boot loaders in their own partitions instead of using a separate System Reserved volume, but I plan to use NTFS compression for each OS, so this won't work. The /f parameter is valid for 10's version of bcdboot but not for 7's bcdboot (I'm using GetWAIKTools to fetch the needed tools for each OS).
The issue with the above, is that it would seem that issuing the commands for each OS, would result in one's boot files/BCD overwriting the other's, necessitating that I use bootrec /rebuildbcd to create a unified BCD that would load both. It turns out that you can't specify a subdirectory with the /s parameter, only a volume letter.
How can I accomplish what I want with just one primary partition, without any unnecessary, complex voodoo methods? Also, it must be something that VeraCrypt can read, so that the OS partitions will be able to mount, so I doubt that using VHDs for each OS's boot files would work.
As an aside, I recently made the switch back to booting from BIOS/MBR rather than UEFI for this particular laptop, since the SSD is only 500GB, so UEFI is more of a preferred but not totally necessary whim of mine, and GPT isn't essential either. So far all of my OSes on this laptop are running just fine on MBR. VeraCrypt isn't UEFI-bootable, and I had more of a desire to be able to freely share my Windows' partitions' data with my Linuxes (which would also enable me to back up and run repairs more easily). Playing around with UEFI loaders like rEFInd and Clover is interesting but not necessary, and in the end I just needed something simple and practical that has been proven to work without a lot of additional configurations, loaders/managers, etc. I will continue to use UEFI on my other machines.