ewilcox I assume your using the standard systemd-boot and not legacy boot? You should just need to smash the F9, Esc, F12, F2, whatever key it is for you to show the boot options and select the one you want.
If you're looking for a grub-like boot menu to choose your boot option, maybe give rEFInd a look.
Edit: For my HP machines, Esc on power up gets my be boot options, on my Lenovo hardware F12 provides the same function.
Edit 2: You can also search the forums, there have been many many discussion about dual booting. Both booting from the same drive, booting from different drives, etc. it's out there.