First convert each ISO to a .imgPTN file and add the two files to the E2B drive.
Then UEFI-boot from the Win7 x64 .imgPTN file (after switching it in) and install Win7.
Then reduce the OS partition to make room for the Win10 OS.
Then boot to the Win10 .imgPTN file (after switching it in) and install Win10.
Note that to make the Win7 imgPTN bootable you will need to add the bootx64.efi file - see https://www.easy2boot.com/add-payload-files/win7-uefi/
Also note that Win7 is very old and does not contain drivers for USB 3 - so you may have problems installing Win7 from a USB drive!
or you could always read my eBook... ;-)
Subject Re: How do I install W7 & W10, as Dual-Boot OS's?