Once the appropriate step in the core rules process is reached, the team that earned the decision chooses where within the a.1-4 turn order play begins. 1. For games after the first game of a match, the losing team of the previous game gets to choose who plays first. If the previous game was a draw, the turn order from the previous game is maintained.
§ 603.7.c Tournament Rules PDF (officieel)
§ 603 2v2
§ 603.7 During matches, teammates sit next to each other in their registered AB order. The setup process for a game of Riftbound is defined in the Core Rules, with these additional exceptions for 2v2: