Joint demand means two or more goods are demanded together. To consume one good, you need another good. In other words they are complements. For example, if the demand for tea increases, the demand for sugar will also increase. Here tea and sugar are complements.
Another good example of joint demand is, printers and ink cartridges. You might have noticed that some models of printers with very high functionality are sold at a very cheap price. You may think that the printers are of lower quality or produced with cheap labour.