There are 2 separate applications that are totally unrelated. Database for App2 is relatively smaller than that for App1. Both are OLTP, both have the same usage window (9 to 5). They both have to be on the same server - so will it hurt to put them as 2 separate schemas under the same Oracle instance or is it wise to put them in separate Oracle instances ? The one reason that I could think of is that I can allocate more SGA to the bigger one if they are in separate instances, what are the other advantages/disadvantages to either approach ?
No, what more reason do you need? Unless you want to complicate your life.
I would store application seperately because of political reasons. i.e different application support/development group and they dont want to share same database resource or they end up blame each other when there are performance reasons
I would not store applications such as Billing Systems and CRM in same instance neither. Resource contending. Unless you implement resource manager