Hi,
This is actually quite easy to do!
Dreams Unlimited (a
Disney travel agent affiliated with this site) sells a 1-day, 2-park Universal ticket with roundtrip transportation from Disney-area hotels. The transportation is provided by Mears, and they'll pick you up in the morning and then bring you back to your hotel at the end of the day.
This ticket needs to be purchased online in advance of your visit. The best part is that it costs $109/adult, which is the same price as the ticket without transportation. It is a GREAT deal...my family did this last year and it worked perfectly. The transportation was convenient and efficient, much easier than dealing with a car for just one day. If you click the "Tickets" tab near the top of this page you'll be taken right to their online store.
Have a great trip!