It is NOT possible as for now to combine this. The system is not et advanced enough to be able to handle it from 2 companies. FLX for trains-and what you here explain as ´valley´ is Off-Peak (Dal in dutch, yes,m that could also mean a low earth area), so you need to get 2 chipcards and keep them apart.
There is a kind of way around, but this is too complicated here to explain.
Welcome to our Community, Myrtoal.
It's definitely possible to combine this.
When you order NS Flex and you already have a bus, tram or metro subscription the system will recognise this and turn off the part of NS Flex that usually calculates your trips for this.
If you have NS Flex but want to order a bus, tram or metro subscription you can contact Customer Service to turn off this part for you.
So you don't need two separate cards.
Or you can get the non-Flex Dal Voordeel subscription, where you travel on balance. That combines just fine with the bus subscription.