Not sure I completely understand your questions.
Bru does back up files sequentially. But the file path is also part of the name. So it would back up in order on each drive. Not all the A’s from all the different drives. Which I don’t even think you can set an Backup job to pull from different locations.
Now one thing that Bru does that drives me crazy is they don’t use the tapes in the library in sequential order. So if a Library Destination has tapes 21-30, It might start with tape 28, then go to tape 30. This really doesn’t cause any issues but I wish I know a way to force it to go in order just for my sanity. In the end as long as it restores it doesn’t matter if it is pulling from 1 and 2 or 1 and 6.