There's only enough for one, so the impression I get is that if say Elena took on the cure and then got drained by Damon, that Elena then loses the cure for immortality and it passes into Damon instead as two people can't physically share the cure? Although I suppose that could still sort of work for Elena as she wouldn't age like Katherine did, so she could still live a full human life either way

And I did wonder why Bonnie didn't think of giving her Mother the cure!