How can I configure a Hyper-V server to use another NIC not which the main server use?

Hello All ,, In simple way you can do the follows steps as below ..
1-Connect new physical ethernet cable on another Lan Port of your physical server .
2-Open Hyper-v Virtual Network Configuration.
3-Add new internal Network , choose the physical another lan card to represent you virtual Network card .
4-The important thing is , go to the another physical lan on TCP/IP properities click on advance and disable the vitual Qeue !!!! this will speed up your virtual lan    Thank You.

from the hyper-v manager you can assinge a NIC for every virtual maschine and you can create virtual switch

When it comes to Hyper V and if you have more than one NIC on your server (if you don't, is a good idea to add), it is recommended to use one NIC to manage remotely the server machine (host) and the other NICs dedicated to virtual machines (guests).
If you added the Hyper V role to the host running a full installation of Windows Server2k8,2k8 R2 so on, you had the option to configure one or more external virtual networks (this is not available when installing Hyper-V on a Server Core); I think the guest will be connected to external networks, however there are also available internal network (if you want to allow communication between guests on the same host; this internal virtual network is not bound to a physical NIC and therefore the internal virtual network is isolated from all external traffic) and private network (if you want to allow communication only between guests on the same hosts).
So, if you want to configure a guest to use another NIC than the host uses, first open Hyper-V Manager, from the Actions menu, select Virtual Network Manager, select External virtual network, Add, give a name, select the NIC you want to use, if necesary, uncheck Allow management operating system to share ..., Apply.
Be aware that network connectivity might be lost.
Next step, in Hyper-V Manager, select the guest you want, Settings, Add Hardware, select Netwtork Adapter, Add, select the network adapter you want, other than the host uses, Apply.
This is how I'll solve the problem, for more explanations you may check http://technet.microsoft.com/en-us/library/cc816585(v=ws.10).aspx.