seeking a confirmation -- SQL Server backup performance
Yes, I tested this with a 2TB database.  The backup time dropped from 5 hours to roughly 4 hours 15 minutes after breaking it up into 4 files and putting them in the same drive.  Another advantage is you could back them up to separate drives if your drives IOPS is your bottleneck.

Also, I believe Paul Randall goes through this in his "Microsoft Certified Master Backup and Restore internals" modules.  

/edit:
Furthermore, I've been testing something similar with RoboCopy's /MT command (Multi-Threading) and we get similar performance gains doing multi threaded copies of backup files.  During our next backup, I'll test to see if I see multiple SPIDs as well.