The ReWire device is a “master” device, this means that Reason has to be opened by Renoise through it.
If you want to slave Renoise to Reason, you have to call Renoise up from within Reason (browse the Reason manual for how to instantiate the other host as a slave host).
Renoise should then detect another ReWire master active and should offer you to start Renoise in Slave mode.