For any one else considering doing this, yes, very easy to do both - I did both, plus lunch in Valladolid and a dip in the Ik Kil cenote in a day last year.

I used Canada Transfers, private driver in an immaculate van (cold water and towels provided, which was a nice touch after CI) and he was totally willing to go off book, stopping at roadside snack places, loo stops he spoke excellent English, gave some colour about the area, history, etc. but was not a tour guide per se. He helped arrange an English speaking guide at CI, we didn't use a guide at Ek Balam.

Found it very relaxing, as we got everywhere before the tour buses did, so all the sites were very quiet