Two networks one broadband

I may be asked shortly to set up a network in an office building.

The office consists of about 7 rooms 4 or so of which will be leased to another company

The office is completely wired with cat 5 ethernet points.

The owner wants to have two seperate networks with neither able to see the other but one internet connection shared between both.

What is the best (and cheapest) way to acheive this? I thought of connecting two switches to a couple of the lan ports on his adsl router, would this work? what are the pitfalls?

Thanks in advance