I worked for the same company in two states, half the year in UT, half the year in ID. I lived in both states as well, but my W2 only shows Idaho where I ended the year. Utah isn't even listed on my W2. How do I file for Utah if its not even listed?

You owe taxes to Idaho, but you have to determine which state is your RESIDENT state, because you claim the credit to taxes paid to the other state on the RESIDENT state tax return.