First question why you are trying to Install two versions ??
Pl check the documentation here - http://download.oracle.com/docs/cd/B40089_10/current/html/docset.html - see "Oracle XML Publisher Administration and Developer's Guide" and "Oracle XML Publisher Report Designer's Guide" on how to use built-in BIP with EBS as an example . Again, a separate install of BIP is not needed.
I am using BI Publisher as a reporting tool for Oracle Billing and Revenue Management (not EBS).
The upgrade documentation is here: http://docs.oracle.com/cd/E21764_01/bi.1111/e16452/bi_plan.htm#BABDIHFB
To enable you to meet the requirement to be on the same platform and architecture, you might decide to install Oracle BI 11g on the same hardware as the existing 10g system. Note that co-locating the 10g and 11g releases in this way would not be appropriate for production environments. Such co-location depends on the availability of sufficient hardware resources and requires careful port management.
Apart from performance considerations, are there any other issues that I should consider when installing 11g on the same hardware as 10g.
I wish to do this to facilitate the migration process, and also to provide ongoing access to the 10g environment whilst testing and validating 11g.