With UEFI you'll never set the hard drive as first device to boot, only the Windows Boot Manager. It seems you already have two listed there, so perhaps changing their Boot order is possible. Make sure you are in fact booting the 8 drive via Windows Boot Manager, if necessary unplugging the Windows 7 drive to test this.
Again try EasyBCD from both OS's. This time in Edit OS Menu tab, delete all listings except the one you are presently booted into. This is how these are reset in EasyBCD, by deleting all but the default and adding the other OS again on the Add New Entry tab using its drive letter as viewed by the drive you're presently booted into.
You can also try to select Load Legacy Oprom on BIOS menu shown above to see what the explanatory text says to the left. You may not need Legacy anything with a fully UEFI bootable system, unless you later add a bootable Legacy drive. If it appears to apply for your BIOS to disable Legacy in favor of fully UEFI then I would try that to see if 8 drive shows up.
If not with WIndows 8 drive set first to boot and both drives plugged in, boot into
Windows 8 Automatic Repair to run it to go over your boot files and check their integrity. This sometimes will also configure a Dual Boot with the other OS. If not try EasyBCD again from both OS's.
If you are certain the BIOS settings are correct then what will for certain configure a Dual Boot is to run
Refresh Windows 8 which will in-place reinstall the OS to overcome any corruption. This is challenging with a lot of factory crapware competing and hindering the OS's operation, so you'd be better off wiping the drive to
Clean Install Windows 8 however if your goal is to get the free
Windows 10 Upgrade Installation and it will run then you can later
Clean Install Windows 10 which will activate automatically from the BIOS that has been permanently given a Windows 10 activation.
You can also follow these same steps in 8 to
Clean Up Factory Bloatware to get better performance and make a Refresh go smoother.