You don't actually have to configure the side button for either, you can just keep it as whatever you have it mapped to now (I assume power menu). Since you can summon either assistant by voice, you can just use voice commands and call the one you want at the moment. Just because the S25 can figure it out doesn't mean that will translate to the lower models. Let's not forget the phone and UI were built together. The only reason I can see for being able to summon an assistant with a button or gesture would be if it was impractical in something like a public situation, or if you are like me and would never say "OK, Google" or "Hey, Bixby".
You should use your power button to turn off your phone, not for some AI stuff. You can however double click it to activate Bixby or Gemini (depending on your preference).
