Federal and postal workers retiring after Jan. 1, 1982 might get two separate benefit checks from the government - one from the Social Security system and the second from their civil service retirement fund.

That, oversimplified, is the possible result of legislation cleared by the House Ways and Means Committee yesterday that mandates "universal coverage" under Social Security for millions of federal, state and local government workers, and employees of non-profits organizations.

Under the bill approved by the tax-writing committee, the federal civil service retirement program would continue in its present form until 1982. Until then, employees would continue to pay 7 per cent of their salaries into the civil service pension fund (with the government also paying 7 per cent) for benefits now worth an average of $670 a month for employees retiring today.

Federal and postal workers - if the bill becomes law - probably would pay in at two different contribution rates to qualify for the same benefits they now get from the civil service fund.

The bill mandates HEW and the Civil Service Commission to come up with a plan, by 1980 (two years before "universal coverage" would begin) that would guarantee that federal workers would maintain their benefits and would not contribute "significantly" more for coverage than they now do. If the plan were in effect today, it would work something like this:

Federal-postal employees now pay 7 per cent of their total annual income each year into the CS retirement fund. Persons covered by social security pay 5.85 per cent only on the first $16,500 earned.

If federal and postal workers are put under universal coverage, committee experts say, they would pay 5.85 per cent on the first $16,500 as do other private employees, plus a 1.15 per cent differential.

That would be a total of 7 per cent Security.

They also would pay 7 per cent on all amounts over $16,500 - maintaining, in other words, their current 7 per cent contribution rate on total income. They would pay more than persons under Social Security alone, but their benefits - backers say - would be at a higher level.

Opponents of the pension merger plan - led by federal and postal unions - don't likethe idea. They worry that the staff pension plan will lose its integrity by being swallowed up in Social Security, which is having serious financing problems. (The CS pension fund has a big unfunded liability, but only because Congress years back failed to make the necessary government contribution. Overall, it is a well-funded, pay-as-you go system. Social Security isn't)

Opponents of the proposal say that the Ways and Means Committee plan is a short-sighted, cynical political ploy designed to pump new money into Social Security and temporarily delay an unpopular tax increase. in the long run, they say, inclusion of government workers could cost the system and in the short-haul it could hurt benefits and raise contributions of employees.