MySQL Replication - 1 Master - 2 Slaves. Inserting works for One, not the other
Have you set your second slave to server_id = 3 (or depends on you, should be different from 1st slave) 

and in mysql of second slave:

CHANGE MASTER TO MASTER_HOST='IP Address of Master Server',
MASTER_USER='slave_user2', 
MASTER_PASSWORD='slave_user_password', 
Below lines will need the values from ‘SHOW MASTER STATUS’ in Master Database
MASTER_LOG_FILE='', 
MASTER_LOG_POS=;