Just because MySQL calls it multi-master doesn't make it so. What you described is single master with a hot spare. Real multi master is when a client can write to both servers and they will be consistent by the magic fairy dust that is replication.

How exactly do you get that multi-TB backup onto the database server's disk so quickly? Even with a gigabit network interface, that's only 125MB/s, which is over two hours[0] to transfer 1TB. Even if you have a 10 gigabit network, that's ~13 minutes[0].

Also consider if the backups are compressed -- a 1TB compressed file also takes time to uncompress. If the data compresses well, you could even run into the disk being the bottleneck, not the network.

[0] Well, more than that; those numbers assume you're actually getting the link rate with no overhead, which of course isn't reality.

Minutes for multi TB restores? Only if both your main and your backup are on SSD, which is still not common for multi TB databases, I think.

1TiB=1,000GiB; if your drives do 300MB/sec, that's still one hour.