Watch the The Jones online guitar lesson by Robbie Calvo from The Rhythm Architect
I wasn't convinced that this piece of music needed another element until I heard the slide playing a lazy kind of motif over it. Quite honestly the two guitar thang was working for me but I do think this guitar part has value and brings a nice tonal flavor which is great for intros and creating memorable hooks.
Hopefully I have inspired you to pick up a slide and at least try laying down this simple part. You can see I'm working around the major chord tones of each chord by moving to that location on the fret board as the chords change each time. Come up with some nice repetitive slide phrases of your own over this progression, it can all be done in standard tuning too, which is pretty cool!