In short, Tokiomi Tohsaka, who knew about the theory of parallel universes, quickly understood this possibility.
So there was no question of him disbelieving it, refusing to accept it, or failing to understand it.
It had to be said that although Tokiomi Tohsaka might not have been especially talented, he was a cautious thinker and widely read. On top of that, he was at least the disciple of one of the world's five strongest magicians—pay attention, the "magic" here referred to the kind that dealt in laws—and an ancestor of his disciple's disciple. That disciple was one of the legendary spellcasters.
Though, as mentioned before, he was the least accomplished of the disciples.
Still, at least most of the "accomplished" disciples had already died out. Only the Tohsaka family remained...
It could be said that there was a reason the Tohsakas were so "arrogant" and insisted on being "elegant."
Though, in practice, they were entirely... "first-rate at dropping the ball, elegant only after that."
Then, drawing on his considerable knowledge, Tokiomi Tohsaka realized that this plane was unusual. It was likely a special kind of parallel universe. He immediately joined forces with Rin Tohsaka—his own daughter.
They had discovered that Gilgamesh's class was Archer, and Rin Tohsaka's Servant was also an Archer. In other words, there might be more than just seven enemies.
That meant they didn't necessarily have to become enemies themselves.
In a normal Holy Grail War, the seven Servants and seven Masters would slaughter one another. In theory, only one Master-Servant pair would remain to complete the "ritual."
In other words, even if they truly became allies, they would eventually become enemies in the end.
But the situation might be different this time.
In theory, if enough Servants died, perhaps he wouldn't have to turn on his