If you remove the "overload" in your nat-statement you will achieve what you want. However, then you would only make the first 6 internal pc:s to communicate since after that your nat pool is fully used.
In your current config when using overload you will keep using the first ip in the pool until it runs out of ports, only after that (after about 65535-1024 concurrent connections) it will start using the second ip.
Remove overload and make sure that your nat pool contains at least one ip per inside host.
/Jimmy