Why can't I start Yadis! Backup twice?
Technically it would be possible to start Yadis! Backup twice, but it would give no benefit.
The main task of Yadis is tracing file changes and making backups. Making a backup of a file - thus copying - requires a lock on the file so the system is sure the file will not change during the copy process.
This means that only one process can access a file at the same time.
Next to this, Yadis is multi-threaded, so it will handle different files in parallel.
Starting a second instance of Yadis would not speed things up as the bottleneck will always be the IO throughput from your source drive to your backup drive.
Multiple instances of Yadis just would have to wait on each other as they are copying a file and consuming the disk IO throughput.