Are these applications production are development (or both) ? Will the applications communicate with each other or are they isolated from each other ? What are the hardware specs for the server and what are the resource specs for the applications (number of concurrent users, type of queries ...) ?
With that being said, I will assume that these are either both production or development applications - If you run both applications under the same instance, you will have greater performance benefits - however, less fault tolerance since an instance treats all databases effectively as one with resources, logging ... If one application has a long transaction which freezes the instance both applications will stop. Most companies will run both applications under the same instance since problems like these are controlled through good design.
The database question revolves around what the applications are and how much interdependency exists - but normally seperate databases. However, since you are asking about putting these 2 applications under 2 instances - it is safe to say to create 2 seperate databases.
Both are production applications and do not communicating with each other at all but both are doing around 100 % of same nature of transactions (number of users, type of queries).
The major reason to run production applications on different instances is if due to some reason (Maintenance, Crashed) I need to down the server then Ill only bring down the particular instance.
The other reason a very huge no of transactions will be served by both applications. For that reason I want to put them on different instances. As you mentioned that on one application may have long transaction that freezes the normal processing of the instance then this approch more practical.
Do I get any benefit to run production applications on multiple instances?
What are the major disadvantages (Performance, Speed, Resources and etc) and benefits?
Well you named all of the disadvantages. The only advantage is in the isolation factor. If you major concern is having to down Informix due to some issue with the application or informix - then go to seperate instances. Some questions to ask are how often will this happen, do you need your applications 24/7, will downing an instance to both applications be unacceptable but one is ok (basically how mission critical is it to have both applications up and running) ...
You will have to decide what are the priorities which will help you make the decision.
If I put my application on single instance then will overall performance dramatically change?
Basically I'm facing problem while implementing HADR. I've successfully implemented HADR on first instance but facing problem while implementing HADR on other instance? So far I'm not sure this is due to multiple instances or I need to fine tune UNIX/Informix system level parameters?
One of our cores utility is not working properly on multi-instance environment but it works fine with single instance.
Does informix recommend multi-instance environment?
Any comments, recmendation suggestion are warm welcome.