Even though it's not always recommended to combine NS Flex with other subscriptions, in this case there is no interaction at all between these two subscriptions and it should work just fine.

Just make sure when you check in for your bus ride the display does not say 'reizen op rekening' instead of 'Randstad Zone Noord maandabonnement'. Because then you will be charged for the bus ride on your NS Flex invoice.

Thank you for the fast answer!  Just in case , that happens “reizen op rekening” appears,  what can i do? 

Then you could ask NS customer service to turn off the BTM Flex option. That will also mean all your other bus, tram and metro rides (the ones that are not included in your Randstad Zone Noord subscription) will be on credit again instead of being billed on your monthly flex invoice. 

Or you could cancel your NS Flex subscription on your current card and order a new one (for free) on a new ov chip card.

But again, this combination should work just fine.

Lets hope it works than!  Thank you so much for your time!  Good evening!

I can confirm that most 'free travel’ (zone/star/region) subscriptions for bus work fine with with NS Flex with BTM active.

Thank you Robert!