Such a custom device would need 2 controllers connectors to be used on an Amstrad plus, but may be implemented directly for a CPC (old) usage.
==Finding proper joysticks==
It is not that easy, especially for the PLUS range.which may make full use of the 2-buttons features (some Cartridges games) or when dealing with modern CPC/Plus games including this feature in gameplay Some CPC era games included a more flexible "redefine keys/controls" option so you could use the second button
Until (but post 1990 games rarely did... and even some Cartridge games where stricts "CPC to cart ports" and didn't even featured the 2 button option in their gameplay.) Or unless you properly rewire a modern oneJoystick.
Not that Amstrad was uncompatible with most Atari Standard joysticks.