Marianne is correct ...
Multiplexing is only there to make sure the tape drive runs at or above it's minimum streaming speed.
LTO 6 drives (slight variation between different brands) have a minimum streaming speed of:
40MB/s uncompressed
160 MB/s compressed.
So, if you are for example backing up uncompreeable files, eg .zip files or .jpegs, you need to send data to the drive at a minimum rate of 160MB/s. I would suggest most data backed up is compressable to some extent, so realistically you need to be sending data to the drive at 160 MB/s to ensure it avoids stop/ start which is extreamly bad for the tape and the drive.
mpx is not there to speed up client backups ... in fact, if we consider clients with compresssable data but the disk speed limts each client to an output of 80MB/s and each client has the same amount of data, and can backup in say 10 mins - it makes no difference to the backup time if the data streams goto a single dive (mpx) of seperate drives - it does however make a difference as to how fast the drive runs.
Multiplexed
Each client sends data to the memory buffers at a rate of 80MB/s, so 160MB/s in total is coming in. The drive is able to write data at 160MB/s, so you have a constant speed of data and the tape drive is happy.
Non - multiplexed using two tape drives
Each client sends data to the memory buffers at 80MB/s, so the client 'data send' rate is the same, and thus the client backup time is the same.
However, the drive can empty to memory buffers twich as fast as they are fillling up, so it has to stop, wait for the buffers to fill, write that data, then stop and wait again - tape drive is sad.
Looking at your figures
stream#1 - 31,747 KB/Sec - finished
stream#2 - 20,825 KB/Sec - finished
stream#3 - 15,972 KB/Sec - finished
stream#4 - 26,760 KB/sec - still running
A total of about 90MB/s - so even using mpx, we only have about 90MB/s coming in, not fast enough for the minimum streaming speed of an LTO6 drive if the data is compressable, and no benifit to splitting the streams to seperate drives. If the speed of the streams combined was faster that the max speed the drive can write at (160MB/s native / 400MB/s compressed for LTO6) then there would be a benefit to using more drives, as at that time, the drive is the limiting factor.