Now that the singular "their" and "they" have gained traction for gender-nonspecivity, this could be acceptable. The card is worded as it is to make it clear that each player receives individual gains rather than both getting the same. Making it "their" may make it more uncertain.
I think that the "is" is missing to keep the text from extending onto an additional line. It's fine as it is as a sentence fragment, but you are correct that it should be there to make the effect a complete sentence.