As of July 10, 2020, the Multi-Gateway feature has been expanded.
It is now possible to connect with multiple gateways of the same provider.
This means you can have two Braintree, Authorize.net or other gateways connected and default one payment type to first Braintree gateway and other payment method to another, etc.
For now this feature does not include gateways: Stripe, GoCardless and Square. These gateways will be available for Multi Account soon.