Ldap string to query multiple domain controllers

Hi,

Is it possible to query two different domains in C# using one ldap string? If so, how would this string be structured? For example, here are the two domains:

1) adext.local

2) ad.com

Currently for the first one my ldap string is (LDAP://DC=adext, DC=local").  Can I add the second domain information into this string so it is queried if the user isn't found in the first domain?

thanks,

Sherazad