Perhaps different types of glass flow at different rates?
A few years ago, I had performed an engineering contract for the Jet Propulsion Laboratory in Pasadena California. My contract was unrelated to this particular event, but I hob-knobed with a couple of the fellows there who told me the following story of their experience. It is hearsay, but these fellows were material scientists and were the responsible parties. Durring the Apollo Missions a window 'glass' was engineered for one of the space-vehicles that was not used when the missions were cancelled. The window was placed into storage on one side inside a frame designed to support it. Over the years the glass 'flowed' downward causing the bottom dimension to swell to the extent that it no longer met mechanical specification...the window was thicker at the bottom and thinner at the top. They told me that glass is a viscous substance. One thing I've seen through the years is in a room full of scientific experts, there is always an intellectual fight about who's right and who is wrong about a 'fact'.
In any case, the two cups, unless the surfaces have been forcibly wedged together by, say, "hands", they will separate, because as I understand things, glass does flow.
plutronus