The answer to this is very straightforward. The two slaves must have the same server_id. I wrote about this 2 years ago (Screwed up replication by sharing server ids). In that post, I quoted Baron Schwartz's blog Pop quiz: how can one slave break another slave.
The quick-and-dirty solution ? Change the second slave's server_id. For example, if the master's server_id is 1000 and first slave's server_id is 1001, go to the second slave and run the following:
mysql> SET GLOBAL server_id = 1002;
This will fix it right then and there.
Then, go to the second slave and change the server_id in the my.cnf
[mysqld]
server_id = 1002
Give it a Try !!!