Eternity means forever. And not just forever going forward. But forever long before this particular lifetime. Twin flames have united and reunited for their spiritual good and gaining, to learn a lesson left unlearned, to resolve something left unresolved, and to continue their journeys together.
They may not show up in each lifetime together. That is why some people feel they have not met their twin flame in this lifetime. This may not be the lifetime for that. However, when you meet your twin flame, it is an unmistakable sensation and knowing.
Your twin flame may not agree with you on everything but underneath it all, you feel that unity. That unity is forever.