It is possible to have two subscriptions on one ov-chipcard, but it is a hassle and not recommended. When you want this, you need to contact Customer Service. In that case you have to use your balance again for journeys outside the validity of your subscriptions, the so-called BTM. e.g. Bus, Tram and Metro.

I do strongly recommend you to use two cards, one for each subscription and you will still have the advantages of Flex. It is allowed to have more personal ovchip-cards. When you subscribe to Weekendvrij, the new ov-chipcard is free.

The U-OV Gemak subscriptions should never be combined on one card with NS Flex. Since they also give discount on train travel in the Utrecht region, disabling the Bus, Tram and Metro part of NS Flex is not enough to ensure both subscriptions work properly.