Networking :: Use Two ISP Lines Simultaneously In Machine?

I have two ISP Connections from different Providers BSNL and Sify. Both are 1MBps. In my Office Now we are using Sify for Only one Machine and BSNL is configured in wifi router. But Often BSNL network is in problem. So we have planned to use both Sify and BSNL simultaneously. what we have to do is to Make some arrangements either in cabling or in configuring we have to use both the connections simultaneously. without hassle so that the employees will always be on-line. and they may not know in which ISP they are browsing. But the internet should be available always.

We have two servers which has running Redhat 5.5 with two Onboard Ethernet card so totally Four Ethernet cards. And we are using Only linux such as fedora, ubuntu and CentOS in our Office and not windows. And all are connecting to internet by wireless. So please give some ideas to use any linux machine as router or something else that would balance the two ISPs equally, that is if one ISP is down then the other should take up automatically without doing any manual configuration and without buying any proprietary software or any hardware.