MySQL / MariaDB - Start multi-source replication with mariabackup (xtrabackup) Many times I successfully performed procedure described in the https://mariadb.com/kb/en/setting-up-a-replication-slave-with-mariabackup/ - it's fastest and easiest because replica is restored by copying files and binlog coordinates for CHANGE MASTER are saved in xtrabackup_binlog_info file.
With multi-source replication it seems to be not possible, because both backups from masters contain their own ibdata1 file - so I cannot restore them into single instance of MySQL (the future slave).
The only way I can think of is like that:

restore 1st master into slave
enable replication for selected databases from 1st master
restore 2nd master into second slave (temporary)
use mysqldump to move selected databases from temporary slave into main slave

The problem is that databases on both masters are large (~1 TB) and mysqldump/restore takes ages - and chasing few days of stalled replication (between backup and restore) takes additional significant amount of time. I'd really like to avoid this.
I know that with MyISAM tables I could just move table files but it's innodb and it cannot be changed.
Maybe there's some way to merge ibdata1 files? Or maybe export/import ibdata1 dictionaries related to chosen databases?
There's no tricks that can be done with ibdata1 files.
Look at using mydumper for parallel extraction and load.
pt-data-sync is a way to replicate the data with inbuilt protections against databsae delay. I don't know if it supports multisource