Walder Frey and Walder Frey

If both players activate the third effect of Walder Frey, who then becomes first player the next round?

The non active player at the stard of the round

The non active player at the stard of the round becomes the first player.

Start of round effects are resolved, as all effects, in order of first player and then non first player.
So if both players declared Walder, the player that starts the round with First Player token would resolve the effect first
His walder would do nothing, as he's already the first player.
Then, the other player would resolve his Walder and become the First player.