Is it ok to re-slope one of my gutters to another gutter?
Maybe.

Generally, the number and size of the downspouts is sized to handle a certain water volume. So if two downspouts were installed, then they probably can't handle just two.

Also, the gutters have to be pitched at down 1/2 inch for every 10 feet. If your facia board isn't tall enough to handle that slope for the entire length, then you need two downspouts.