There's a command to "move units as stack." Your faster units will have to sacrifice some of their movement points at the end of the turn, but they'll be on the same tile as the slower ones. It's either X or J or Ctrl-X or Ctrl-J (I forget which one), and there are buttons on top of the square at the lower right corner that will allow this. Hover your cursor over the various buttons there and a description of the button's command will pop up.
Edit: X-post.
Edit: X-post.