There is no good and safe way to do what you're asking without 2 amps or a two channel amp which might be already there. That would also probably be the cheapest solution to what you're asking about. A powered mixer would be a possible solution if it had more than a mono powered output, which most do. You could use the "left" for outdoor and the "right" for indoor as an example.
The best solution would involve a lot more work and money. Installing a "Constant Voltage System" usually a 70v system in the US, but it could be a 25v or 100v too. they have "zones" where you can change the volume per zone. That is something you really want a professional setting up.