multi-residency vs. single instance, multi-database

We are getting ready to implement PeopleSoft Financials 7 and HRMS 7, phased in over the next year. We are running Informix Dynamic Server 7.30.UC3 under HP-UX 11. Our initial plan was to use Informix' multi-residency capabilities to have one database instance for Financials and one database instance for HRMS, both running on the same box. Our thought was, we could tune the engines separately and any problems which required one instance to be brought down would only affect that instance and not the other instance. However, recently we are beginning to question the performance of two separate instances compared to creating a single database instance with two databases -- one for Financials and one for HRMS. We're concerned that the cost of two competing instances for one set of resources may require more overhead than creating and tuning a single instance. I'm curious to know people's opinions on this topic (preferably for any other PeopleSoft customers, but really anyone who has had experience balancing two application databases on a single machine). Another alternative is to have each instance running on a separate machine, but based on constraints, we will have to run them both on a single machine for at least the next year. Thanks in advance for any ideas/opinions/comments. Chris Corey ccorey@icrr.com Database Administrator Illinois Central Railroad