After the roster is frozen a player can only appear on one roster per age division. Below is from the USSSA rule book:
3.05 Player Roster Eligibility:
3.05.A Prior To Being Qualified / Frozen To An Official Online Roster (age Divisions 4U – 14U ONLY):
3.05.A.1 A Player can only play on one team in any given tournament and can only play in one tournament at any given
time.
3.05.A.2 Players shall only physically play for one (1) team per day, event, tournament, week or weekend, regardless
of location, venue, age division and / or classification.
3.05.B After Being Qualified / Frozen To An Official Online Roster (Age Divisions 4U – 14U ONLY):
3.05.B.1 3.05.B.2
3.05.B.3
Players shall only appear on one (1) Official Online Roster per age division and shall only participate with such qualified / frozen team (unless previously released pursuant to these rules).
Players participating in multiple age divisions shall only appear on one (1) Official Online Roster per age division and shall only participate with such qualified / frozen team(s) (unless previously released pursuant to these rules) but shall not participate on any team below the classification of the oldest team in which they participate on.
A Player can only play on one team in any given tournament and can only play in one tournament at any given time.