To the original question: unfortunately, previous posters are correct and your options are a bit limited. If I remember correctly there's at least one shuttle service that will take you between the two, but because they have set arrival/departure times I found the inconvenience outweighed the savings when I looked into that option years ago. It'll be about $40 each way for a taxi or half that for Lyft/Uber, and I've found that to be the most straightforward way to handle a single day's trip out to Universal from the Disney bubble. You could also rent a car...but it didn't come out any cheaper when I looked at it for a 2015 trip, and driving in Orlando isn't fun.
Personally, I'll go against the grain in this thread and say that my preference is to stay at WDW, in no small part because I'm always going to spend more time at the WDW parks than the Universal ones. Having done a WDW trip where we took two day trips to Universal, though, my preference in future would be to do a split stay if I were planning to do two or more Universal days. It's a longer drive than you might expect and I found it a bit annoying to have to bring a full change of clothes and rent a locker in case we got wet, and then to have a long taxi ride ahead of us at the end of the day--and once you go multiple days the cost of transportation starts adding up. Better to start at UO for two or three nights, enjoy the proximity to the parks, and then only have to take a cab once when you transition to WDW for the remainder of the trip.