It really helps if you learn the proper draw at first when learning to shoot with both eyes open. If you draw properly and correctly, your sights will line up perfectly, and your dominant eye should automatically train itself to see the sights clearly, and after a little while, you'll be able to just pick up the gun and point, and see straight through the back sight, and see the front sight with both eyes open.
Proper draw technique if you want to practice it, is:
GRIP: Grip your weapon, and at the same time bring your support hand to center body mass, right in the center of your chest with your hand cuffed as if you're expecting to grab the weapon with it. How your hand sits when you're supporting your grip hand while firing is exactly the way your support hand should be when you bring it to your chest.
CLEAR: Clear the holster of your gun (pull the gun out of the holster, until the muzzle has cleared the top of it)
CENTER: Bring the gun to Center body mass. It should meet up with your support hand at this point
EXTEND: Extend straight out, and your sights should line up perfectly.
Practice that, and eventually you will learn to shoot with both eyes open. You might have to move the gun around a little bit the first few times to get it right. It helps to take it slow at first. Practice drawing slowly, in the 4 steps.
Other than that advice, it just takes time and practice
If you see double images (I still do) shoot at the one on the right, and youll be shooting directly at the target. Ive heard that if you turn your head to the left a little the double image will go away, but it doesnt work for me. A way to make sure that you're infact looking through the sights properly with both eyes open, is to close your left eye, and you should see the sights plain as day lined up with your right eye only