I agree, the answer is no.
It may be possible to merge the client file indexes, but not the media database. Unless you can do both at the same time, then there is no point.
Your alternative is to decide which backup server will be the production one, then keep the other one running just for recovery purposes of old data, at least until the data passes their retention time.
The other option is to take the tapes from one backup serverr, and scan them into the backup server you want to use.