The throttle lock is down on the front right side of the left throttle. I'll get a video of it in action and post for everyone either tonight or tomorrow. It's not too difficult to get to with one hand. It uses a magnet so it's easy to slide in and out of the right throttle's locking port. The left throttle has the small metal magnetic stud in it that slides right / left in and out of the right throttle to lock or unlock them.
I found it easier to just use them both unlocked in the P-38 and move them together. The friction on the left and right is identical, so you don't input any unintentional asymmetrical thrust.
Those threads I linked in the original post, although lengthy, answer many questions.