Yes - once you sell the card - you no longer have the privileges... so you mint it - activate the features, enjoy them... and THEN you can sell it if you like.
or you can keep it! The features will be able to be reactivated every 6 months!
And yes - if you have 2 Ultimate quill cards - you get to use 3 features on both!
but since most are available for only one month - it makes more sense for you to use one card in one month - and then use the next card for the next month!
Does that answer your questions? if not... ask more! hehehe