The local public bus system for the Orlando area is called the LYNX system. It has buses that go to Disney World (to the Ticket and Transportation Center) and buses that go to Universal Orlando, but no bus that connects the two. I don't think you can even get between the two with only a single transfer. There is a also a tourist bus called the I-ride Trolley that goes somewhat near (but not to) Universal, but doesn't go near Disney at all. I join the others in recommending you pay for an Uber, Lyft, taxi, or other paid service to transport you between your Disney Resort (which I saw is AoA in your other post) and Universal for the one day you plan to visit there. If you were planning to do multiple days at Universal, I would recommend a split stay.