If you need the 2 cables for the amount of traffic that is crossing you could think about a LAG connection where multiple physical cables are set up as one virtual connection.

MST is an option where you can have multiple different "instances" of spanning tree running.  You can define specifically what VLANs are associated with a certain instance.  You would have the capability of sending traffic over a certain path for one instance.  Then for the other instance you can send the traffic for a different VLAN thru a different path.  This can get complicated pretty fast.  You may want to read up as much as you can before trying to implement this in your production environment.

Page 554 of the CLI Guide provides configuration commands for MST.

Page 352 of the User Guide also talks about the configuration options.

Here is a link to the manuals available for the 6200 series:

downloads.dell.com/.../powerconnect-6248.html

Here some more information on configuring MST on a 6200 switch:

www.dell.com/.../pwcnt_MSTP_interoperability.pdf

I hope this helps,

Let us know if you have further questions