Thanks!
You have the right idea, that the Get Available Times element is for a single User/Schedule - so in this case we need to have a Get Available Times element (or Workflow Action) run for each of the service providers.
You can do this with a repeating group on page and using the Orchestra Plugin to run this data in multiple Get Available Times elements at once, more about this is described here
Or the same logic can be done using a Recursive Loop and the Workflow Action Get Available Times - running through each provider to get there availability, and then filtering out if there is a time on the searched time
Another option that might be desired if Searching via Time Slots is a major feature of the app would be to actually save Available Time Slots directly as Data for the schedule, then searching would be simpler and faster. But then you need to make sure you are properly updating the Available Times with every Booking, Cancellation, Schedule change with a backend workflow