Run D6 and D7 instance on same domain?
Put the D7 site under the root web directory. You should be able to go the site and see the D7 site. Then, put the D6 site under the root web directory in a new directory called 'd6' or whatever you want. Then, in the settings for the D6 site, put the root path as webdirectory/d6.

Then you can reference the d6 site from the d7 by directing users to the link "something.com/d6/someNode"

Otherwise, you really can't have both running under the same web root. It has to choose one or the other.