2 x FMS
There's nothing very mysterious about using two FMS. If they're configured to synchronise automatically at startup then you just put whatever you want into one FMS and it's transferred to the second one. This works for the NAV, Flightplan, and DTO pages but the fuel pages are independant which means you can run different fuel scenarios on each unit. you can also manually put them into independant mode.
We use IAS for the entire climb but that will be in your Ops manual presumeably.