How can I find what was taxed by both states (Idaho, Missouri) on my W2?

State tax withheld (if any) is listed on your W-2 Boxes 15, 16 and 17. . 

Box 15 shows the state

Box 16 shows how much of your income was taxed by that state 

If the question is how much was taxed by BOTH states-

Add the amount taxed by Idaho and the amount taxed by Missouri. 

If the total is more than your total wages earned, the difference would be what was taxed by both states. 

For example, if you earned 50,000 total and Idaho taxed 30,000 and Missouri taxed 30,000 then 10,000 was taxed by both states. 30,000 + 30,000 = 60,000        60.000 - 50,000 = 10,000 

If the question is how much was taxed by EACH state, the answer would be 30,000 EACH