Two Macs sharing same Internet connection

Hello!

I have a specific problem about two Macs sharing the same internet connection.

When my Mac was the sole machine I used, I had no problems.

I have a Static IP provided by my ISP because one the Macs hosts four websites and I wanted to eliminate any problem for customers to find my sites.

My router set as Bridge and my connection to the ISP set up in Network Preferences.

Till now everything was running fine.

Now I have a second Mac, more powerful which I use as a "workhorse" for heavy graphics tasks but for which I also require an Internet connection.

Problem is : My customers can't visit my sites as their requests aren't directed to the right Mac.

How can I solve this problem?

Many thanks in advance for your valuable advices and tips!

Gerard

PS: I have an Airport Extreme which you can include in your proposed solution if necessary.