The Bixby button can be customised to perform different actions with either a single tap or double tap. However, you can only do one or another. If a single tap is assigned to launch another app, a double tap will open Bixby Home and vice versa.
Samsung however has blocked the Bixby button from being mapped to Google Assistant or any other AI voice assistant. This was confirmed by Dan Seifert from The Verge on Twitter. It seems that Samsung is really keen on pushing Galaxy owners to get acquainted with Bixby Home. With that being the case, many users have resorted to workarounds that remaps the Bixby button. XDA-Developers confirmed that there is a workaround. Using a third party app called Tasker, users can create a dummy app that launches Google Assistant which can be mapped to Bixby. This not a definite solution but it works, for now. (Source: Android Authority, 9to5Google)
