I want to install Windows 10 on External SSD and then be able to move it between 2 Macs, is that possible?
While technically possible, it would be exceedingly cumbersome because each time you booted, Windows would consider itself to be running on new hardware and require a new activation and new device driver installations. See https://support.microsoft.com/en-us/help/20530/windows-10-reactivating-after-hardware-change.

A better option is virtualization. You can run vmware, parallels, or the free https://www.virtualbox.org, installed on each of your macs, and keep the virtualmachine file on the external drive.