Most NAS devices use an ancient implimentation of Appletalk (i.e. classic appletalk ala netatalk). This is _much_ slower than what OSX uses (appletalk over tcp/ip).
Couple of suggestions to try. First, try connecting to your NAS with smb:// protocols instead of afp:// .. you may be surprised at the results. If that doesn't work then you will probably be better off with the drives attached to a machine for sure.
Lastly, you can try doing some media streaming if you want to get really geeky
I use VLC ( http://www.videolan.org
) and do realtime transcoding to UDP streams targettted at the IP address of the machine in question. I can actually get away with doing an decent quality UDP stream (DIV3 video @ 384kbs, MP3 audio @ 96kbs) over my VPN and high speed internet access, and watch videos from my media server from remote locations all the time
Going with this approach you could potentially run much more than 2 streams without bottlenecking (well maybe at the CPU of the streaming server hehe)