goodroi - 3:00 pm on Jan 12, 2009 (gmt 0)

I am not sure if I understood your question. I think you are asking if you refer to another domain in a robots.txt file will it help improve rankings. In my experience there will be no significant ranking improvement and you are probably increasing the chance of breaking your robots.txt file.

Most robots.txt files have no links pointing to them and thus they have no significant link popularity to pass. The only possible benefit I could think of is that there is a chance that the search engines may increase the indexing of the domains. I do not see how referring to another domain will help pass along a significant ranking boost or make the search engines think the site is an old, trusted authority site. In my opinion it would be much more beneficial to increase the quantity and quality of your inbound links for both domains.

If you were asking a different question please let me know.