Usually the only time the first basemen is involved as a trailer is when a ball is hit down the right field line with a runner on 1st for a play at the plate. Runner on first and a ball hit the RC gap you still want your 2B and SS as your double cut guys with the SS being the trailer. 1B would still follow the runner to second base or be in the area between the pitchers mound and first base.
Usually the only time the first basemen is involved as a trailer is when a ball is hit down the right field line with a runner on 1st for a play at the plate. Runner on first and a ball hit the RC gap you still want your 2B and SS as your double cut guys with the SS being the trailer. 1B would still follow the runner to second base or be in the area between the pitchers mound and first base.