Connect the Windows 8.1 SSD to your motherboard's first SATA port (usually labeled SATA0 or SATA1).
Connect the Windows 7 SSD to your motherboard's second SATA port.
Connect your DVD drive to your motherboard's last SATA port.
In your UEFI (BIOS) setup, make sure that the first SSD (Disk0, Windows 8.1) is the primary boot device.
Insert your Windows 8.1 DVD, reboot, and press your UEFI's boot device selection hotkey repeatedly. When the boot device selection dialog comes up, select your DVD drive, making sure to choose from the UEFI section if given a choice.
Reinstall Windows 8.1, removing all partitions from both SSDs when you get to the drive selection screen.
Once Windows 8.1 is installed, open File Explorer and rename C:\ to "Windows 8.1".
Open Power Options -> Choose what the power buttons do -> Change settings that are currently unavailable, and disable Fast Startup (very important to prevent disk corruption when dual-booting).
Insert the Windows 7 DVD and reboot, following the same procedure as before to boot from the DVD drive. When you get Windows 7's installation drive selection screen, be sure to select "Disk1: Unallocated space", which is the other SSD. You should also see "Windows 8.1" as Disk0 with several partitions.
When setup automatically reboots your computer, Windows 8.1 should ask which OS to load. Be sure to select Windows 7 so that setup can continue.
Once in Windows 7, open Windows Explorer and rename C:\ to "Windows 7". D:\ should be "Windows 8.1" as labeled earlier.
You should be good to go! Please note that Windows 7's boot time will be longer than usual because (at least last time I checked) Windows 8x boots quite far before asking which OS to load.
then i simply added a command line in the bcdedit when i was in Windows 7:
i first checked with diskpart list vol, what letter was assigned to the Windows 8.1 installion (g:\)
so now the system boots to a Windows 8.1 menu style, asking which os i wanna start.
Windows 8.1 loads instantly, while Windows 7 takes a while, not sure why exactly.
"You should be good to go! Please note that Windows 7's boot time will be longer than usual because (at least last time I checked) Windows 8x boots quite far before asking which OS to load." -- might be because of that.
it's interesting to note that, the windows 8.1 boot menu can let us decide which OS we want to load by default. So picked up windows 7 and then the boot menu reverted to an old style classic dual boot menu. Now windows 7 loads pretty fast
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