It might if you do it right. The XHD utility lets you RAID 0 (STRIPE) or RAID 1 (MIRROR) your drives. RAID 0 Will give you a bit better performance, but if one of those drives dies, you will have to reformat the second one (I.E. no redundancy). RAID 1 will mirror the first drive to the second drive, so you'll have an exact copy in the event of a crash.
http://en.wikipedia.org/wiki/RAID#Standard_levels
If you're having a bottleneck at the HDD, have you considered running an SSD or a faster HDD like a VelociRaptor 10k?