Hi,
After some days of searching and tests, as usually the solution is simple:
In the VMware Server, just remove the network adapter from one of the machines and then just add a new network adapter.
Then, configure the network (ip address, gateway, dns), because de new network adapter comes unconfigurated, restart it, and that's it, now the two virtual machines can work simultaneously with internet access. To add a third or fourth machine, just repeat the procedure.
Like i sensed, this was not a SME Server issue but rather one related with VMware Server.
The cause of this problem is simple:
I've created a base installation of a virtual machine with some definitions and programs i like, so when i need a new virtual machine i just make a copy of that base installation and start from there. Because the two virtual machines are equals, some network issues arise, i believe, related with the mac address.
So, applying the solution above does the trick.
Regards