There should be coming soon an update to Bixby which includes option to independently map the single tap and double tap of the Bixby button to either Bixby or an app. Supposedly google assistant is the one app they won't let you link to. I think there's ways around that: create a tasker task to launch google and convert it to an app using App Factory.
If you have the update, the option for remapping your Bixby button will show up in your Bixby settings as he shows here
I don't have it on my AT&T S9+ yet, even though I have Pie. If I go to Bixby Settings / About Bixby, it tells me I have the latest Bixby. Go figure. I think it'll come... eventually.