Fun with numbers just because. According to this website for a local glass manufacturer, a sheet of clear, untempered glass that is 30 inches by 60 inches by 0.25 inches weighs about 37.5 pounds, or 167 newtons. (I used the approximate size of my window, and 0.25 inch thickness is about standard for single-pane windows.)

The area of the glass is 1800 square-inches, or 1.16 square-meters. If the space between two pieces of glass is an exact vacuum, then the difference in pressure is 1 atmosphere, or 101,325 pascals. So the total force due to air pressure on the glass is about 12,000 newtons, which is about 700 times the weight of the glass. It won't be an exact vacuum, so let's just say the difference in air pressure is 0.5 atmospheres. That would still make the force due to the air pressure about 350 times more than the weight of the glass.

So if the two pieces of glass were placed on top of each other exactly and you applied a force perpendicularly to the glass (imagine lifting up the top piece with a really big suction cup), you would effectively have to lift up about 351 sheets of glass to separate the two pieces. Of course, this is not how you would separate them in practice. You would like slide the top piece past the bottom piece first. That way, more of the top piece is exposed to the atmosphere on all sides. That is, the force due to air pressure is F = PA, where P is atmospheric pressure and A is the area of contact between the two sheets of glass. If you slide the top piece, the contact area decreases, thus decreasing the required force.

Obviously, you would never separate the glass by applying perpendicular forces. Just slide the top glass until it is totally separated from the bottom piece. The force required is just F = mW, where m is the coefficient of friction between the two glass pieces (very low) and W is the weight of the glass sheet. But it's fun to know that it's not just your imagination that it is very difficult to simply lift one piece of glass off the other. In fact, removing a high-quality suction cup from a bathroom wall is also much more difficult than it seems like it should be. Now you know why!