James M. Bowie thinks you should throw out your old toothbrush -- andbuy one of his newfangled ones.

Bowie's company is the U.S. licensee for Action 2, a two-headed, Y-shaped monster of a toothbrush that cleans both sides of the teeth at the same time and supposedly reaches places that rarely feel a toothbrush. To use it, you slide your teeth between the two facing banks of bristles and srub back and forth.

Bowie calls the Action 2 "the most significant development in hygiene since dental floss," and he expects to clean up with it, predicting that half the people in the United States will be using a two-headed brush within five years. Not everyone is quite so enthusiastic.

"It's just another gimmick," says Harry Bohannan, research professor in the University of North Carolina's dental ecology department. It is not the first dual-headed brush, he says. The firs one introduced at least 30 years ago. Moreover, Bohannan complains, it is cumbersome. After trying it, he says he "can't anybody would use it for very long."

The forked toothbrush has found its way into plenty of mouths around the world, though. Since 1981, customers in other countries, including Australia, West Germany, South Africa, and Japan, have bought more than 2 million Action 2 brushes, according to Jeff Chelin, comptroller of Action Hygiene Products Inc., aToronto-based company with world distribution rights to the product. That is a lot of brushes, but no more than a small bite of the American retail market: close to 200 million brushes a year.

So far, Oranamics Inc., Bowie's Atlanta company, has sold about 150,00 tooth-brushes, one-half to dentists, dental schools, and other professional markets. retail sales, at a suggested price of $3.98, began late last year, and Bowie says he expects the brush to be in 5,000 stores by the end of the year.