Once more please....
 

"A pitcher (P1) starts a game. He is replaced by pitcher (P2). Pitcher (P2) is taken out and (P1) comes out and throws warm up pitches. The Coach decides to put (P2) back in and let him pitch before (P1) throws to any batters the second time. Is this legal? Does P1 have to pitch to a batter and can P2 be put back in if P1 does not pitch to any batter and only warms up?"

Thanks in advance.

In OBR, P1 must pitch to a batter until the batter is retired or reaches base (or the side is put out). He is now a substitute pitcher.

May be different elsewhere.

NFHS 3-1-2

If a pitcher is replaced while his team is on defense,the substitute pitcher shall pitch to the batter then at bat,or any substitute for that batter,until such batter is out out or reaches first base,or until a third out has been made.To ensure that the requirments of this article are fulfilled,the umpire will deny any coach-denfensive player conference that would violate the rule.

Note. A player may be removed as pitcher and returned as pitcher only once per inning.