White House officials are discussing the possibility of replacing the National Endowment for the Arts and the National Endowment for the Humanities with one or two independent agencies, possibly modeled on the quasi-governmental Corporation for Public Broadcasting.

Nonetheless, arts community observers have been speculating during the last few days about how and why such an agency would work.

"It's one of a number of ideas," said Aram Bakshian Jr., special assistant to the president in the office of public liaison for the arts, humanities and academia. "There's been a lot of wide-ranging discussion. A lot of people from the outside have had ideas. But until something is really decided, no one can talk about it. We're not ready to make a comprehensive statement."

According to two sources, if the arts endowment were turned into an independent agency, it could be run by a part-time chairman (who would hire an administrator) instead of by a full-time chairman, as it is presently. It has been rumored widely that if that course is taken, actor Charlton Heston would be offered the chairmanship.

"I haven't heard from Mr. Reagan or from anyone in the administration," said Heston, reached yesterday by phone. "I did talk to [Attorney General] William French Smith before the inauguration. I told him when they got around to it, I wanted to talk about the arts -- just to provide some input and observations. But no one has talked to me yet."

Heston is president of the American Film Institute and a former member of the National Council for the Arts, the presidentially appointed body which advises the National Endowment for the Arts.

About the possibility of a job as chairman, Heston said, "I suspect they will be talking to me, but it seems far down the road."

Heston added, "I could not take a full-time job with the administration."

If he were to be approached about a part-time chairmanship, Heston said, "Well, we'll see. . . They'd better do it pretty soon because I'm going on vacation in Canada for a few months. This is what I do for a living. All this other stuff is free stuff," he quipped, adding, "Oh, if I was offered something, I would treat it seriously."

Plans for an independent agency are still vague and it is not clear at this point if there would be two agencies -- one replacing each of the endowments -- or one.

According to sources, the idea of an independent agency originated with New york lawyer Barnabas McHenry, the general counsel to Reader's Digest and to the Wallace Foundation in New York. McHenry, who sits on the board of the Metropolitan Museum of Art in New York, took the idea to his friend, James Baker, chief of staff to President Reagan. Baker discussed it with Edwin Meese, counselor to the president, who brought it up to Nancy Reagan. She reportedly discussed it with the president while he was in the hospital.

Since then, a variety of White House-connected people have been involved in developing this idea and others. More than a month ago, McHenry had a meeting with National Endowment for the Arts Chairman Livingston Biddle. The topic of discussion was how the endowment worked, "a fact-finding mission," said one source.

An NEA spokeswoman said yesterday that Biddle "is unavailable and doesn't want to comment. If he's asked at the [House appropriations subcommittee] hearings, he'll talk about it." Those hearings are scheduled for early May.