First you have to install mod_proxy & mod-rewrite.on windows or many linux ditributions these modules are installed by default, so you may found them on C:\Program Files\Apache Software Foundation\Apache2.2\modules
in windows and /usr/lib/apache2/modules
in linux.
so you need to load them via apache config:
unix-like os:
LoadModule proxy_module /usr/lib/apache2/modules/mod_proxy.so
LoadModule proxy_http_module /usr/lib/apache2/modules/mod_proxy_http.so
LoadModule rewrite_module /usr/lib/apache2/modules/mod_rewrite.so
windows:
LoadModule proxy_module modules/mod_proxy.so
LoadModule proxy_http_module modules/mod_proxy_http.so
LoadModule rewrite_module modules/mod_rewrite.so
then configure two virtual hosts on your apache:
Listen 80
NameVirtualHost *:80
<VirtualHost *:80>
ServerName iis.mydomain.com
RewriteEngine on
RewriteRule ^/(.*)$ http://localhost:81/$1 [P]
</VirtualHost>
<VirtualHost *:80>
ServerName apache.mydomain.com
RewriteEngine on
RewriteRule ^/(.*)$ http://localhost:82/$1 [P]
</VirtualHost>
for more information please see here and here
EDIT 1:
An article about using apache on windows