My question is if there anyway possible to merge many Raspberry Pis to improve performance?
(eg. Run one OS in two raspberry?)
You can't "just" install an OS on 2 Pi's. (it is possible, bear with me)
If you want to start to use multiple Pi's you set foot in the world of distributed computing.
There are many examples of people running clusters of Pi's.
Is it useful for any kind of production environment? no
Is it fast (as in, strapping 2 pi's together makes it one fast machine)? no
Does it have any practical use outside of learning purposes regarding distributed computing? no
If you want to learn more about distributed computer I would suggest starting with a VM provided by a major Hadoop vendor, should you choose to go the hadoop route (i.e. Cloudera/Hortonworks/MapR). You could also have a go with some Docker containers, or have a look at Apache Spark or Apache Mesos.
But to come back to the original question: Yes you can, but no you can't 'just' install 1 OS on multiple machines. In case you aren't completely put off by my answer yet, and are still interested in some kind of datacenter OS: Mesosphere provides this (this is a proprietary Apache Mesos project).
This guy is compiling Mesos for ARM, so you could have a look there.