So is this mythic Tarrasque equal to two CR 30 normal tarasques like Tiamat? And it is a mythic template-applied to a normal tarrasque? Also, why use normal CRs instead of increasing the CR past 30? And how exactly does the mythic template work?
So the Mythic template is from Theros.
Monsters with Mythic are normal (usually legendary) monsters until you reduce them to 0 HP. When you reduce them to 0 HP, they remove all conditions, regain their legendary resists, regain a whole pile of HP, and gain access to their Mythic actions.
These mythic actions can be normal, reactions, bonus or legendary actions.
The new Mythic monster is designed to
also be a CR X encounter (roughly as hard as the first one you fought). So encounter-building wise, you fight a CR X monster, then another CR X monster right afterwards.
This way you can have a "non-Mythic" fight with a monster, or you can have a "Mythic" fight where the monster "transforms" half way through.
A problem with exceeding CR 30 is you start having "single shot kill HP damage" levels of damage and the fight changes phases from "can you fight this without ever letting the monster connect" instead of "can you fight this".
A double-length fight, meanwhile, keeps up the reasonable per-round threat of a CR 30 monster, and just makes it take longer.