So, you and your twin brother both have horrible cancer. The worst cancer. But a fantastic doctor invents a magical cure, which unfortunately has a dread side effect... They suck all the cancer out of one of you, but they have to pump all that cancer back into the other, until one of your is healthy and the other has double cancer.
Tuvix and Tuvok both want the same job. Who get's it?
Tuvix and Neelix both want to fuck Kes. Who get's it?
There can only be one.
B'Elanna is a Transporter Clone and no one cares.
That would have been a nice twist to Barge of the Dead, making a couplet to Faces.