How to setup a MacBook Pro with DHCP Server over USB WiFi dongle
To get that working you have to be able to create an ad-hoc network with one of the two Wi-Fi interfaces. It works at least with Airport but only without any Wi-Fi security like WEP/WPA/WPA2. I don't have access to a RTL8188cus, so I don't know if its driver allows a "safer" wireless networking.

Additionally you have to install a lightweight DNS/DHCP service like dnsmasq.


Open Terminal.app
Install homebrew:

/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

doctor homebrew:

brew doctor

install dnsmasq:

brew install dnsmasq

copy config and plist files:

cp /usr/local/opt/dnsmasq/dnsmasq.conf.example /usr/local/etc/dnsmasq.conf
sudo cp -fv /usr/local/opt/dnsmasq/*.plist /Library/LaunchDaemons
sudo chown root:wheel /Library/LaunchDaemons/homebrew.mxcl.dnsmasq.plist

create some additional config files and folders:

sudo mkdir /usr/local/etc/hosts
sudo touch /usr/local/etc/hosts/hosts.conf
sudo mkdir /var/lib/misc
sudo touch /var/lib/misc/dnsmasq.leases

Create an ad-hoc network with your Airport Wi-Fi interface including a name.
Open System Preferences -> Network -> Airport Interface and configure an IP-address, a netmask and a DNS-server manually:

IP-address: 192.168.2.10
Netmask: 255.255.255.0
DNS-server: 127.0.0.1

Modify /usr/local/etc/hosts/hosts.conf with nano:

sudo nano /usr/local/etc/hosts/hosts.conf


Content (append a trailing empty new line):

127.0.0.1       localhost
192.168.2.10    host.foo.bar www.foo.bar

Modify /usr/local/etc/dnsmasq.conf with nano:

sudo nano /usr/local/etc/dnsmasq.conf


Content:

...
# Add other name servers here, with domain specs if they are for
# non-public domains.
server=/foo.bar/127.0.0.1
addn-hosts=/usr/local/etc/hosts/hosts.conf
...
# If you want dnsmasq to listen for DHCP and DNS requests only on
# specified interfaces (and the loopback) give the name of the
# interface (eg eth0) here.
# Repeat the line for more than one interface.
interface=en0 #modify the interface name if necessary
...
# 3) Provides the domain part for "expand-hosts"
domain=foo.bar
...
# Uncomment this to enable the integrated DHCP server, you need
# to supply the range of addresses available for lease and optionally
# a lease time. If you have more than one network, you will need to
# repeat this for each network on which you want to supply DHCP
# service.
dhcp-range=192.168.2.50,192.168.2.100,12h
...


If you need further settings/options read the comments in the config file.
load the dnsmasq daemon:

sudo launchctl load -w /Library/LaunchDaemons/homebrew.mxcl.dnsmasq.plist

Configure your web-server properly.
Start your web-server.





Connect the client hosts to your ad-hoc Wi-Fi network.
The clients have to get their IP-addresses on the Wi-Fi interface via DHCP. It usually doesn't work with manually configured IP-addresses (without changing them or only by accident!).
Open a browser on the client hosts and enter www.foo.bar.