There isn't any little trick or anything that will make you have a single, clear sight picture as that is the physical reality of having two reference points of vision as opposed to just one. Sorry if I'm restating the obvious.
This is just wrong. You need to see an ophthalmologist if you can not focus both eyes on a single point at arms length and see only one of the object you are focusing on. As you get older the ability to focus on an object at arms length or nearer starts to diminish at about age 35 and continues to get worse until about 65 for most people when it stops getting worse as its about as bad as it can get, but you should still be able to see a single blurry opject. This is called presbyopia and is fixed with reading glasses for close vision.
You cannot focus on both the front sight and the target at the same time, this is true, same for your eye as your camera.
(Actually I can, because of Lasik, my left eye focuses from infinity to about 4 feet, my right eye focus is from about 2' to 6', but this doesn't work very well because of parallax -- a consequence of have two eyes spaced apart. After 8+ months post-surgery I'm still experimenting with what works best in a given situation but any of what I try is better than what I had pre-surgery!)
When you are young enough you can switch the focus distance fast enough to think you might be able see near and far at the same time. I lost this ability by about age 30.
With iron sights you will shoot most accurately when you focus on the front sight and align the blurs of the rear sight and target about this focused point -- either one eye shooting or two. With red dot optics you look thru the dot and focus on the target.
When you focus on the target with iron sights you are "point" or "instinctive" shooting, which can be highly effective form 3-7 yards with moderate practice and up to 15 yards or so with serious practice.
For SD, I practice point shooting from 3-7 yards and don't even bother to raise the gun at eye level, shooting as soon as I feel pointed on target after the draw. You are trading accuracy for speed here, no doubt about it. If you have a safe place to practice it, its worth doing. An Airsoft gun can be a good training aid here.
--wally.