Headley may be quizzed by Ranas lawyers on his several wives
Rana`s counsels said they had received a voluminous number of e-mails authored by Headley, including a portion that made reference to his past relationships.
They asked a federal judge to make prosecutors turn over other e-mails written by 50-year-old Headley.
"Headley had several girlfriends, wives, etc. in the period," Charles Swift, a Rana attorney, was quoted as saying by `Chicago Sun Times`.
Headley is expected to testify against Rana at a hearing due here next month.
In seeking more e-mails, lawyer Patrick Blegen said Rana`s team sees relevance if they show Headley keeping secrets and selectively disclosing things to his friends, including Rana.
"We want all of them so we can decide," their relevance, Blegen said.
But Assistant US Attorney Daniel Collins said the government has already turned over what is relevant.
While Headley has a wife and kids who live in Chicago, he also was married to at least one other woman at the same time.
The New York Times had reported in October last year that two of Headley`s three wives had approached the US authorities about a possible terror attack on Mumbai before it was carried out in 2008, killing 166 people.
Headley, who met Pakistani-Canadian Rana as a youth at a military school, has pleaded guilty in the case and provided information to authorities.
Rana, who is being held in a federal lockup pending his May trial, is accused of aiding Headley`s efforts as he went on scouting missions to Mumbai as well as in a separate plot involving a Danish newspaper.