Two Instances on Multiple CPU Machine

Hi , I have a quick couple of questions. We have a SUN machine with SUNOS having 4 CPUs. There is already one instance of Informix(7.2x) up-and-running which uses 2 cpus (1 and 2) by setting AFF_SPROC etc and all other parameters. The second instance of Informix has been configured to use CPU number 3. There is also an MRP application running, so we figured, we might leave one CPU for that application. Now the question. Can we get any performance improvement/degradation if we configure our second Informix instance to use say, two CPUs ? Also if both instances are configured to run on say, three CPUs each, will there be any positive/negative impact ? What would be the optimum allocation (of CPUs) in the above scenario ? Thanks Madan