what is dual onboard lan?

Answer 1

That means you have 2 network cards that has been built on to your computer. When it has been built-in, it is no longer a "card".

Open your computer and have a look...

You normally only need 1 network card but if you want to build a cluster, it will require 2 network cards.

Cluster Requirements as outlined by Microsoft.
http://www.microsoft.com/windowsserver2003/ccs/sysreqs.mspx

"All nodes require at least one NIC. If a private network is planned, the head node requires a minimum of two NICs to create a public and private network, depending on the network topology selected.

Each node may require more NICs as appropriate in case of public network access or in support of an MPI-based network."