Question: I want to know how to install two bluestacks in a system and then run it.

Is it possible to run bluestacks in Virtualbox

Answer: 
**Running Bluestacks inside VirtualBox works.**

Out of curiosity I tried to do what OP asked: I created a windows 8.1 pro virtual machine, downloaded bluestacks, installed it and it just runs. The VM has 2048 MB RAM, 1 CPU Core and even without installed virtualbox guest tools speed isn't as bad as you might think. The host machine was a Intel i7 3770K (Ivy Bridge) with 16 GB RAM. So YMMV. But at the time it also had bluestacks running on the host and two instances of android with genymotion, so I think it should be fast enough for many configurations.

**Running multiple Android devices on your PC is much easier with genymotion.**

I don't know what you want to achieve but if you want to have multiple android devices at the same time on your PC, you could do this with genymotion much easier than having bluestacks inside a virtualbox machine.
You can run directly android on VirtualBox I have tested and it's working very well. the version 4.4 r2 is a good one
You can find some iso [here](http://www.android-x86.org/)