These were made by 3rd party vendors as a replacement to the original JY-2 joystick for the Amstrad CPC computers.
In order to push sales of Amstrad's own joysticks, Amstrad designed the CPC with one joystick port only which had two different ground levels. The idea was to sell one joystick which forwarded the second ground signal to a built-in joystick port, to which the second joystick could be connected.
As the quality of the Amstrad joysticks was lousy, using an Y-cable together with other joysticks was the preferred solution.