Acting doubles in ''Josh and S.A.M.''

In the recent Josh and S.A.M., two brothers well under the driving age steal a car in Texas and head north to Canada. But a hard-hitting investigation reveals that-get this-the young actors behind the wheel of the Pontiac Grand Prix aren't really driving! Notice when Josh takes over the wheel for Sam (or ''strategically altered mutant,'' as his brother calls him) on a dirt road. The driver seen pulling away in a long shot sits about six inches higher than does actor Jacob Tierney in the following close-up. If that isn't enough, Josh's denim jacket, worn over his hooded sweatshirt, disappears after the first shot in the sequence. Talk about strategic alteration.