Why are double glazed windows useful in summer

Unfortunately, you cannot salvage the pane that is not broken. You will need to order a replacement of exact measurements or bring the sash to a glass supplier so a new sealed and gas filled unit can be made. A double pane that is not sealed will collect condensation and fog up quickly. Not an option.

Usually this is a sign that the seals have failed and moisture is getting inside the window. The video you posted seems to be of a service that can apply a solution to prevent the fogging, however the seals are still gone which means the window is no longer insulating as it should be.

Depending how new they are, they might still be under warranty in which case you should contact the manufacturer. Otherwise you can contact a glazer to see what would be involved in replacing the glass and seals, though sometimes it's cheaper just to replace the whole window.