Luckily, Dave's answer isn't entirely correct. Fusion is perfectly capable of presenting two IP addresses behind the one authenticated Wifi Session session.
Only problem is that most routers will not issue two DHCP leases to the same WiFi MAC.
If you really need your guest OS to function as a server over WiFi (i.e. accept inbound connections) the simplest way is to assign it a static IP address within the local sub-net of your WiFi network, then select "Bridged" mode.
There are other ways which involve creating port-forward rules by manually editing config files, but because they've Macified the GUI in an effort to make things simpler, there is no way to do this via the user interface.