When Selectors battle, there is a large wheel or clock that decides which Selector goes first.
The Selectors are represented by their deck colour (eg. Yuzuki - Red Deck, Ruko - White Deck) The image below is from when Ruko battled Yuzuki, And it's clear that Ruko gets to go first.
But how could we tell who goes first if both Yuzuki and Ruko had the same coloured deck? Would the wheel be entirely one colour?
As of season 1 of the anime there have been no same-deck battles but maybe there is some external source of information about this?