We've gotten so many questions, so we'll just put this here - Double Bike-Rafting is a thing. Or at least since we invented it during EO2020 course designing in May 2019. The below is an image of Stephen Thompson and Jason Magness preparing for the "test" - a 42 mile stretch of river with lots of class II/III whitewater.
Jason took a cheaper "Ghetto bike" on the front - just in case of disaster, but Stephen is seen strapping on his 29er race bike to the back. Note, we also experimented with having both bikes stacked up front which can work.
The boat model is the Alpacka GNU RACE, but we imagine this would work well on the Forager or even the old style Gnu (but maybe with less ideal paddle stroke).
Regardless - the rule should be PRACTICE. If you are renting boats, and will not have time to do this at all before the race, we wholeheartedly recommend using singles, as they are (in general) more forgiving to noobs.