timbur, adding a second nic or using firewire for MP communications is very easy. My secondary machine has two Gb NICs connected to my LAN and I use the firewire as the interface for MP communications. All works just fine.
On a side not, you said that you can't use split tunneling; what does it have to do with MP? You shouldn't have any problem communicating with you local PCs even while connected through VPN because the VPN interface is a "virtual" one briding over you physical nic, which leaves your physical NIC also available for internal communications within your home network.
The only thing is that if you use the Microsoft builtin vpn client and need to send packets over a gateway (ie to go on the Web), if "Use default gateway on remote network" is checked, then you won't use your home Internet connection but will rather be redirected to the gateway to which your vpn connection points to. Unchecking it mean you'll be able to go on the web from your personal Internet connection and still be able to communicate with your corporate network.
One important thing to note though, it's that splitting connections isn't very secure, unless you use a firewall on your laptop that can protect against the security hole it creates between you local Internet connection and you vpn tunnel that points to your corp network. That's why some 3rd party or vendors vpn clients won't allow such setting. But with Microsoft builtin client, you should be able to do it easily, unless you are tied to some group policies that restrict this (which many admins don't really enforce anyway, for practical reasons).
Hope this help.
