J.H is correct, but in the fact he has come back with a second post he has demonstrated something - This needs planning carefully and is not really a 5 minute job to do. Also, I personally don;t think there is enough information.
For example:
1. What is the data (eg. oracle DB, regular files, large files, very small files
2. How fast can you read the data off the disk
3. How are the 2 LTO4 drives connected, that is, how many connections are coming out the back of the server.
4. How is the data layed out on the disks
If you have say 4 directories 1,2,3,4 but they are on the same physical disks, attempting to backup more than one area at once will hit performance, as the disks will be maxed out.
If (for example) dir, 1,2 are on separate spindles to 3,4 you could backup 1,2 as one job and 3,4 as another:
New Stream
/1
/2
New Stream
/3
/4
By then reducing the number of jobs allowed to each drive to one, two drives will be used, with data from each drive coming off different disks.
If the SAN can support this throughput, then that is good.
Another question is how fast is it going at the moment. I presume you are using only one drive.
If this is ging <40MB/s (approx) then you can't even drive one drive fast enough, and would perhaps need to consider multiplexing the data from different disks but just to one drive, to get the average speed up.
50TB is a large amount of data, running to just two drives. How much time do you have for this to complete. If this is a "single backup" then you may wish to consider using checkpoints also.
Martin