Yes you can have both the ORACLE_HOMES and the databases running off them active at the sametime. The 2 users can be connected simultaneously to the 2 ORACLE_HOMES at the sametime too.

While installation you do not need to have to ORALCE logins for the the 2 ORACLE_HOMES, but yes you will have separate listener.ora files, etc 'cause it will monitor the connections to that particular environment.

I am sure you can find some documentation on multiple ORACLE_HOMES on metalink.

Hope this answers your questions.

Chintz