you can always Query the intermediate table to get the required values. In CRM when ever a N:N relationship is created there will be intermediate table created which will have lookup for both the records. in your case the table name could be something
line "new_lead_new_customentity" or new_new_customentity_lead". this entity will not be accessible using advance find but you can always write your own fetchxml/Query Expression to retrieve records.
If my response answered your question, please "mark the response as an answer" and also "vote as helpful". Regards, Hari. www.crm2011byhari.blogspot.com