UA is in terminals A and C (A for flights serving former CO cities, and C for flights serving former UA cities), while US is in terminal B. These are in separate buildings, and inconvenient to move between. There is no airside connection. You can see a map at
http://www.massport.com/logan-airpor...tive-maps.html
There is a free shuttle bus that goes between the terminals. It's also possible to walk, although I don't have personal experience with that.