Since this is a HS site, I always state the NFHS rule first and as such there is some difference in rules from NFHS, NCAA and OBR in this rule.....
NFHS rule 1-1-4
Except the catcher, all fielders must be in fair territory at the time of the pitch. A fielder is in fair ground if he had ONE foot in fair territory.
NCAA rule 5-4
Except the catcher, all fielders must be in fair territory when the ball is put in play. A fielder is in fair ground if he had ONE foot in fair territory.
So one foot in is all that is required in NFHS / NCAA games.
OBR 4.03(professional baseball and all leagues using OBR)
Except the catcher, all fielders must be in fair territory when the ball is put in play.
Official Interp.....A fielder is in fair ground only if he has both feet in fair territory......however umpires should not insist a fielder have both feet in fair territory unless one team complains but thereafter the umpire will insist that both teams fielders have both feet in fair.......
also to add confusion, in "Pro" baseball the official interp is that first basemen do not have to have both feet in fair.......
so depending on what code your game was played under those are the choices........if you have umpires who call multiple codes, you will most liekly be ok with one foot in.