The CLI manual defines the behavior for multiple dynamic gateways. I would guess multiple static gateways behave the same way -- that is, if both segments containing gateways are currently directly connected, they will have the same weight unless there is a way to define that weight manually, and it will be some arbitrary choice, but if one if the segments goes down, it won't be chosen.
From the manual:
Usage Guidelines You can use this command to set the default gateway to the IP address of the interface on the upstream router or switch to which you connect the controller. If you define more than one dynamic gateway type, you must also define a cost for the route to each gateway. The controller will first attempt to obtain a gateway IP address using the option with the lowest cost. If the controller is unable to obtain a gateway IP address, it will then attempt to obtain a gateway IP address using the option with the next-lowest path cost.