Carcassonne: two builders on the same city?
At any time when the builder is on the table, it functions as documented in the rules.

The number of builders in a single city does not affect the way in which each builder functions. Nor does the number of knights. Only the active player gets an extra play if he/she extends the road or city. A builder can be deployed even if there are thieves, knights, or builders of other players on the road or city, as long as you have a thief or knight present.

Furthermore, the builder is not counted when calculating the majority. This is because the builder is not considered a follower.

Only if, due to expansions like The Tower or The Dragon, a player's last follower is removed from a road or city, the builder will no longer function. At this point, you're supposed to return it to your supply.

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