Some of these posts aren't for the route you want, yet I think there might be useful information. You'll want to check how many hours it is by bus between those two cities. Our Mendoza - Santiago bus trip was 7 hours or so, including border formalities. It was a wonderful trip, but I wouldn't have wanted to tack on Santiago to Mendoza. It sticks in my mind that is a 12 hour ride.

Start with this post and read subsequent ones in the same thread for very useful information.

Here is another excellent post.

Here is my post about the bus from Mendoza to Santiago.

Here is a TripAdvisor post about the bus from Buenos Aires to Santiago.