OK, here is how I installed Batocera X64 to a Surface 2 Pro. It has one internal SSD. What you Need:
Two (!) USB Sticks with 2GB each.
An USB hub.
A different PC that can build the Images for the two USB sticks.
Etcher (balenaEtcher as it is now called) and Rufus (both are USB Tools for Windows).
Stick #1 - create a Batocera stick with Etcher, it can directly write the .gz Image to the stick
Stick #2 - a bit more complicated - create a Clonezilla stick with Rufus based on the instructions on Clonezilla.org
Insert Stick #2 into PC and boot (on a Surface this requires turning off Secure Boot!). Clonezilla is not a userfriendly piece of Software and I am too lazy to write complete instructions. But once Clonezilla is up, insert Stick #1 into the hub. Then you can do a drive-to-drive Imaging from the USB to the internal SSD.
This worked like a charm for me, the Surface now boots directly into Batocera and I have 110 Gigs free for ROMs and more.