No, one database can never be mounted by more than one instance! It is the other way around: one instance can have two or more databases opened at the same time, but only if this is paralell server configuration.

In tnsnames.ora AFAIK there is no ora_sid entry. In There is only SID=... in the CONNECT_DATA entry of SQL*Net version 2.* and Net8 and this means SID as instance name, not database name. In 8i this was changed, there is no SID entry, it is replaced by SERVICE_NAME=..., which in this case means global database name (if service described in this tnsnames section is a database).

HTH,