cisco 3550 switch two isp two different lan

I am having a cisco 48 port 3550 switch with L3 image. I am having Two ISP with static IP from both.
121.240.153.0/24 – VSNL
121.240.156.0/24 – RAILTEL.

Now my problem is
one dept should use VSNL,
other Dept should use Railtel.

There should be no mix up between the two ISP.
How to configure the switch to do the above configuration