I don't think you can make the authentication scheme dynamic, but you can have two applications that share the same cookie name in the authentication scheme, and redirect accordingly. ie - one app is really thin, just there to accept the alternative scheme, and redirect back to the main app.
Do you think it should allow to select which authentication scheme to use in the login page in future versions of Oracle APEX? I think this is very common in many Websites today? Thanks.
I have success with the following link and you also have to enable the setting "switch in session" within the authentication scheme. Don't know if you be able to get this to work from an outside link i believe the session ID has be created to be able to switch. Dimitri Gielis has a nice blog on how this done within his world cup 2018 app, if you search for it you should be able to find it.
f?p=&APP_ID.:1:&APP_SESSION.:APEX_AUTHENTICATION=<name of authentication scheme>