Use 2 Windows-Versions from external disk

Hey everyone,

I want to have Win 7 and a Win 10 on a bottable drive.
From what I know, I can only use a single Windows installation per Partition.
As a USB-drive can be used on different PCs, how can I configure EasyBCD to boot from another partition on the sam external disk?
In generall i will not now the drive letter, will I?