Overtraining is a myth.
Overtraining only occurs when you don't get enough recovery time during the WEEK. Not during the actual lift period itself. For example, lifting the same body part every day of the week balls to the wall THAT is an extreme example of overtraining. overtraining is not allowing your muscles to fully recover during the week for a variety of reasons including: lack of sleep, lack of rest time, lack of protein, ect.
You could exhause your muscles to the point where you cannot move them and you are NOT overtraining. It only becomes overtraining when you do not give them time to recover as the week goes along.
The harder you work your muscles in the gym the longer it takes for them to recover. If you work your muscles moderately then moderate recovery time is needed. If you work your muscles HARD then a lot of recovery time is needed. If you do 2 sets and leave the gym then basically no recovery time is needed. It all depends on how depleted your muscles are.
That is the rundown on "overtraining".