Here is the progress of the question:

March 3: Oracle816 server was installed on ORA-HOME1 on Server1. Database ORK1 was created. Configured tnsnames so that SQL PLUS can connect to ORK1 on server1 and ORC2 on server2.

March 5: Some Database Admin tools like EZSQL and PLSQL Developer were installed on Server1. Everything OK.

March 22: Oracle Enterprise Manager with Tuning Pack was installed on ORA-HOME2 on server1. The installation also created a seperate tnsnames file.

Now, the DB tools can not connect to ORK2 on server2. Probably because the new TNSNAMES created on March 22 causes some problem.

Question: if multi-Oracle Homes have multiple TNSNAMES, which TNSNAMES will DB admin tool select to use? Can I change it?