I'm gonna be very real with you all. They could have used the transporter and/or time travel to duplicate tuvix and have both survive
Tuvok and Neelix are trapped inside every version of Tuvix, desperate, pained, and suicidal, screaming for their freedom.
Remember in the Lindsay Lohan version Of Freaky Friday when mum and daughter ran into each other as fast as they could, hoping that their switched personalities would snap back to normal..
That would not work with the real crew, but it probably would with the quicksilver crew from the Demon Planet, if they really wanted it hard enough.