Google Tag Manager set field value to one of two variables

Question

We are using Google Tag Manager. It is sending a custom dimension to Google Analytics with a value of {{fc-isSubscribedSession}} variable

The fc-isSubscribedSession variable is a DOM variable that is taken from the data-is-subscribed-session attribute on the page body.

We are migrating our platform. The old user interface makes use of the tag and the data-is-subscribed-session attribute. The new user interface is more like SPA so we are using the data layer to push the data to GTM like

window.dataLayer.push({
    'newInterfaceIsSubscribedSession': current_user.is_subscribed?
  });

(above is a fragment that is close enough to the real call)

Now the problem is that we must fill the dimension from fc-isSubscribedSession or from newInterfaceIsSubscribedSession.

Do you know if there is a way to fill a dimension from one of two values?

Something like {{fc-isSusbscribedSession}} or {{newInterfaceIsSubscribedSession}}