"Basque chess - A chess competition in which the players simultaneously play each other two games on two boards, each playing White on one and Black on the other. There is a clock at both boards. It removes the bonus in mini-matches of playing White first. Basque chess was first played in the 2012 Donostia Chess Festival in the Basque Country, Spain. Also called Basque System." https://en.wikipedia.org/wiki/Glossary_of_chess
One you play as black and the other white.