from www.sharepointhack.net:

Earlier I spoke about how you can tell the people-picker control to look up a list from a different domain.

Now what happens if you need people-picker to look up users from two different domains and each domain has a separate account that needs to be used to view users?

The answer is quite simple; you can append another domain with a separate user account to the "stsadm -o setproperty -pn peoplepicker-searchadforests" command:

stsadm -o setproperty -pn peoplepicker-searchadforests -pv domain:domain1.com,domain1\account1,password1;domain:domain2.com,domain2\account2,password2 -url http://url.com

Note the semicolon between the two domains :)