This is quite common in old buildings in cold areas such as northern Europe, the northern United States, and Canada. Two single pane windows are place a short distance apart. There are also "storm windows," a similar concept where a new window is retrofitted to the outside of an existing single window. Both windows are usually easily openable for summertime ventilation.

The outer window can acquire significant condensation, especially if it's fully sealed. This can drip down and rot the wood sill. The outer window is not supposed to be fully sealed. There's supposed to be an air gap at the bottom so that this doesn't happen, but some installers or homeowners are unaware of this and caulk all around.