Two Montgomery County real estate companies -- Bogley Inc. and Noah & Cummings Inc. -- have affiliated with Mount Vernon Realty Inc., one of the larger multi-office real estate brokerage firms in Northern Virginia. It is Mount Vernon's first venture into Maryland.

Bogley, a 40-year-old firm headed by Robert W. and Peter Lebling, has offices in Chevy Chase, Potomac and Kensington. Dean Noah and Beale Cummings head their firm, which has offices on Rockville Pike and in Gaithersburg.

Mount Vernon president W. Howard Rooks said the new affiliation was neither a merger nor an acquisition. He said the new linkage with "firms already established in the Maryland market place" will fit in with Mount Vernon plans to open more offices in this metropolitan area.

The association, Rooks added, will bring the number of Mount Vernon offices to 25 and the number of sales agents to approximately 800. Mount Vernon is associated with Relocation Resources Inc. and also has a guaranteed sales program and a time-sharing department for resort properties.

The association was formed at a time when the resale market has been depressed by high mortgage rates. Generally, brokerage sales have been lower in 1980 and 1981 than in the latter part of the 1970s, when resales set records here and in most parts of the nation.

In the past year Merrill Lynch Real Estate acquired both the multi-office Chris Coile brokerage in eastern Maryland and the Colquitt-Carruthers brokerage that had about 30 offices in the Washington area.

Several years ago the multi-office Routh Robbins firm was acquired by Coldwell Banker, which recently dropped the name. Both Merrill Lynch and Coldwell Banker are among national realty firms that have fought the trend to national franchising identification of locally owned offices (such as Century 21, Red Carpet and Realty World) by establishing chains of their own offices in large cities.