MCI Communications Corp. yesterday reported that its second-quarter revenue was nearly double that of a year ago, but profits grew only half as fast.

The company announced quarterly revenue of $363.8 million, up 96 1/2 percent from the same quarter last year. Net income climbed to $54.6 million (47 cents a share) from $36.5 million (38 cents) in the same period a year ago--a 49.7 percent increase.

The Washington long-distance phone company has embarked on a major capital expansion plan to try to take advantage of the coming breakup of American Telephone & Telegraph Co., and MCI said that is one reason why earnings aren't growing as fast as revenue. "I would characterize this quarter, and perhaps the entire fiscal year, as one of gearing up for the divestiture," William G. McGowan, MCI's chairman, said in a prepared statement.

Analysts expect MCI to keep increasing its own growth and market share. "We're continuing to recommend the company aggressively," said Steven Chrust, a telecommunications analyst with Sanford C. Bernstein in New York. "We also expect a favorable rule on access charges this fall, and the prospects for equal access in the long run are gangbusters."

In addition to its long-distance service, MCI provides international telephone service, mobile telephone, telex and data transmissions.

Security National Bank, the holding company that operates Security National Bank, reported second-quarter net income of $819,000 (89 cents a share), a 14.1 percent increase over second-quarter net income of $718,000 (78 cents) in the same period a year ago.

Net income for the first half of 1983 totalled $1.52 million ($1.66, up from $1.50 million ($1.64).

The corporation's total assets increased by 22 percent to $189.997 million as of June 30.

First American Bank of Maryland reported net income for the second quarter of $765,000 (77 cents a share), an 18.4 percent increase over $646,000 (66 cents) for the second quarter of 1982.

For the first half, First American of Maryland earned $1.46 million ($1.48), up 19.3 percent from $1.23 million ($1.26) a year earlier.

As of June 30, the bank's assets totaled $373 million--up 16 1/2 percent from the preceding year. The level of loans increased by 5.1 percent, and the Silver Spring-based bank said deposits increased by 22.2 percent from the same period last year to $330.4 million.

National Bank of Washington had profits of $834,000 (61 cents a share) during the second quarter compared with a loss of $401,000 (29 cents) during the second three months of 1982.

For the first half, NBW reported net income of $2.17 million ($1.58) compared with $107,000 (8 cents) a year earlier.

The bank's assets averaged $1.05 billion in the second quarter, down from $1.07 billion in the second three months of 1982. Deposits grew to $820 million from $806 million a year ago.