I have two network on a PC, one is for our online software and the another for Internet. But the problem is that at a time I can use only one network. Even both are displayed as connected it's of no use. How can I use both networks at a time ?

What you want to do is called load balancing, or network teaming. Load balancing will allow you to specify what applications or services use which connection. You can say, P2P stuff on A, Video on B, surfing on A, etc. Network teaming is a little different. That lets you connect two separate adapters and then you can share bandwidth across them by convincing your computer you have only one adapter (virtual adapter attached to both connections).

I think load balancing can be installed in windows via adapter properties. Teaming I've heard of, but have never seen. I was talking to a coworker about something like this a while ago, he suggested that the adapters must be dual on-board, or at least the same models from a company that supports this. They will come with the software, not all network cards have this capability.