You can map both drives at the same time - in fact you can map multiple folders on each drive to multiple drive letters - as many folders as you have unused "drive letters" for.
This is purely a Windows function - in Windows 7 ...
1 - open libraries
2 - right click "Network"
3 - select "Map network drive" from the resulting menu
4 - select an unused drive letter
5 - click Browse
6 - navigate to the DNS-323 and expand it
7 - select the volume (Volume_1 or Volume_2)
8 - you can either click OK at this point which will map the volume or you can navigate down to a folder.
It's pretty much the same in any version of Windows, except that Microsoft has changed the names of certain things - libraries was once called explorer, network was once my network places.