Are you trying to boot W10 from the W7 bootmgr ?
You cannot boot a newer version of Windows using the bootmgr of an older version.
The older bootmgr will not recognize the digital signature of the newer Winload.exe as valid.
You should change your BIOS to boot from the W10 drive and add an entry for W7 to the W10 BCD.
The newer bootmgr is completely backward compatible with all earlier versions of Winload.