If you're flying Ryanair, as long as your return flight is within 15 days of your attempt to print a boarding pass, you can print outward and return passes at the same time. I can't understand why you're unable to, unless you missed the box which needs to be checked asking if you want to check in for your return flight, too.
EasyJet is even better in allowing boarding passes to be printed 60 days in advance of travel. I can't understand why airlines like BA will not let you do this until 24 hours in advance.