There is no easy answer here. I think the most correct answer would be RATHER NOT.
There is rather no performance to be gained but running multiple instances of the same app in multiple tomcats on the same physical machine except for:
* you need a lot of heap per session. In this case you will probably be able to save gc time and performance. So in case you need more than 12 Gb Heap separation would make sense.
* you have multiple physical resources your app can't use properly. For example if you could give each instance its own database or its own file system.
* you have concurrency issues in your application.
- Database. If you have one. You will have more connections and evtl. more locks. And both are limited resources.
If you want a more detailed answer, you should tell us a bit about your app. Or, better, you start to monitor your app, run one physical server with one instance and another one with two and compare.