Those are basic steps:
1. Hardware need to be the same, this is basic HPC requirement, so using two different servers is not recommended and you, probably, need some kind of job scheduler - SLURM, Torque, LSF, etc. If you are not using identical hardware, you most likely will have an issue related to performance.
2. Use the same network adapter ( onboard Ethernet, HighSpeed Adapter aka HCA). See item 1.
3. Install same OS, drivers
4. Is using IB, be sure to run OpenSM
5. If you are using Mellanox hardware, use HPC-X toolkit - http://www.mellanox.com/page/products_dyn?product_family=189&mtag=hpc-x
6. Run jobs
Your simple question, is is compilated subject that included almost everything - fabric design, networking, performance tuning including BIOS, OS and driver ( for reference you may check Mellanox Tuning Guide) and it is definitely should be splitted to different subjects.
Take, for example, this article - http://hpcugent.github.io/vsc_user_docs/pdf/intro-HPC-windows-gent.pdf (135 pages)
I would suggest to start building and run jobs using onboard Ethernet adapter ( for HPC you can use any communication channel that exists on the host). When this phase is over, add Mellanox and you'll get much better performance.