Can Ivern play 2 Brushes at the same spot?

No, you cannot play two Brush tokens at the same spot.

According to the Green Father legend card, its ability allows you to "replace that battlefield with a Brush battlefield token." Because a Brush token is a battlefield, it occupies the battlefield slot. If a battlefield is already a Brush token, there is no original battlefield left to "replace" with a second Brush token, and you cannot have two battlefields occupying the same space simultaneously.

Additionally, Rule 438.1 defines "Replacing" as creating a token in the place of another card or token. Once a space is already occupied by a Brush token, there is no valid target to perform a second "Replace" action on that same spot.