I too am learning to keep both eyes open. I can do it fairly well when skeet shooting, but handgun has been more difficult for some reason. But I agree with the others, it's a practice deal. I am getting better but there are days when it's worse for me. Maybe light conditions or mental block...who knows?
I like the pen idea. you can practice that in the bank or grocery store, where the dry firing may get you in deep trouble
Below is a tool that may work like the tape method that Lycanthrope mentioned above, but not sure if it's the same deal or not. Take a look.
Lyman's new Eyeglass Diopter
instantly improves eye focus for shooters with prescription glasses. Eliminates target fuzzing and distortion. Mounts directly to the eyeglass lens, rubber suction cup will not scratch lens. It is the ideal product for shooters whose eyes simply don't focus the way they used to.
It's kinda goofy looking and I think I have seen shooters using a homemade version of this on the skeet fields. Don't know if this would help with keeping both eyes open or not. Take a look
. (Is that a pun)?