I've got a better answer than my first one.
Give the guest a single Host-only network adapter, and enable Internet Connection Sharing (ICS) on the host. I've tried this on a Windows XP host with a Windows XP guest.
The guest can connect to the Internet.
The guest can connect to the host at an address like 192.168.0.1 (chosen by ICS). -- Remember to allow the guest through the host's firewall.
The host can connect to the guest at an address like 192.168.0.22 (assigned by the DHCP service provided by ICS).
