Need Help on where to install the Bootloader after installation

Hey guys, I am new here

I have 2 hard disks, one is WD Blue and the other is Seagate

I didn't have a lot of storage on the Seagate HDD, so I had to install macOS as well as windows on WD Blue

I created an EFI partition on both hard disks of size 500MB each, I did not want to mess with windows EFI, so I copied OpenCore Bootloader to Seagate HDD EFI, Later I wanted to format a partition on Seagate HDD to APFS and I was stopped by failing to unmount the disk

I was able to format the same by booting into the USB Installer

I wonder if this is happening because I have macOS and OpenCore on different hard disks

Please Help