Connecting two LANs

Hello,
I work in a small education institution which operates on a very tight budget. Currently we have switch based LAN connecting about 50 PC, and shared a single 2MBps DSL internet line. We are planning to add another 2MBps DSL internet line to the network. My goal is to divide this LAN into two separate subnet (for better management), with each subnet having their own 2MBps internet connection without losing connectivity with each other. Here are the simple diagram:

Subnet1—switch1—-router1—–internet

subnet2—switch2—-router2—–internet

My question is:
What is the cheapest solution (free is possible) to connect subnet1 and subnet2.

Thanks.