Telephone customers in Maryland will be able to list any two family members' names free of charge in the telephone directories instead of just one as a result of a Maryland Public Service Commission ruling announced yesterday.

The ruling, the first of its kind in the Washington metropolitan area, will enable the 1.5 million Maryland customers of the Chesapeake and Potomac Telephone Co. to list the names on the same line without an additional charge, the PSC ruled.

The dual listing service previously cost 40 cents per month for each additional name, a C&-P spokesman said. The service will take effect Friday, the spokesman, Webb Chamberlain, said.

Although there will be no charge to individual households requesting the dual listing, any extra costs incurred by the company eventually will be passed back to all phone users, he said.

Although the company originally had requested that a one-time-only charge of $8 be imposed for the service, Chamberlin said he did not know how much, if at all, phone bills would rise as a result of the ruling.

A similar request is being considered by the Virginia State Corporation Commission and is expected to be voted on in April, Chamberlin noted. No such action is currently beign considered in West Virginia or in Washington areas also served by C&P, he said.

"The change is a matter of the times," Chamberlin said. "Until recently individuals and groups had not requested the service," he said.