There are hidden picture books which look like random noise until you manage to focus your eyes 'behind' the book. I could never do that but others could. So I would say yes, you can with some effort.
I mean with just one eye, being able to consciously refocus that eye.
For example, looking at this writing on your screen able to make that go in and out of focus.
I can bring my finger into focus, drawing my focal point in front of the screen, and if then remove my finger I can hold that focus (with about a 50% success rate). Perhaps that is cheating a little ;).
I can do this. It took me a very, very long time to figure out how to explain it to people who have "normal" vision. For what it's worth, I also have diplopia and I'm convinced that they are related.
This image[1] is pretty good. Look at the two squares, and squint a lot, so you see double, and the try to make the pretty far. Now gradually squint less and try to make the two center squares align with each other so you see three squares. When you manage to do that you have the correct focus. Shift your gaze to the image.
