Multisite would be the easiest way to achieve this – assuming both sites are WordPress. There are other ways to achieve the same but they are more complicated.
You’ll find the instructions here:
http://codex.wordpress.org/Create_A_Network
Related Posts:
- How to use alias domain for multisite installation?
- How To Add Custom Form Fields To The User Profile Page?
- Why are my roles not visible in a Multi-site/Network?
- How to use same email for multiple users
- Where can I find documentation on what characters are allowed in user names and why?
- Can a multisite’s blogs be accessible from two different sub-domains?
- WP Multisite development with Mamp Pro and wildcard subdomains, not really working for me
- WordPress Multisite allow site admin to add user without email confirmation
- SSL wildcard issue for wp multisite subdomains
- What is the best way to load the WP environment in a subdomain of my multisite WordPress install?
- Wildcard subdomain for the same site
- Benefits or Problems for using Sub-Directory vs Sub-Domain install of WP Network
- Different back-end colour scheme for the different sites of a multisite
- Set up collaborative site
- Transfer Subdomain site to Multisite setup
- Multisite installation combining subdomains and subdirectories
- Prevent user creating new users with specific roles
- “Preview Changes” button doesn’t show changes for previously published posts/pages
- Changing Multisite URL after the subsite has been in use
- How to add multiple existing users to a multisite site?
- How can I get multisite primary blog (url or path) for current user?
- How do I maintain static directories on multisite subdomains?
- Changing admin sub-domain in WordPress multisite
- Subdomain login problems
- How can I have multiple URLs pointing to single WordPress installation?
- How to Redirect login User with Mapped Domiain Primary blog Url to Original Sub-site Url
- Multisite setup on IIS with subdomains
- WP Multisite – domain mapping to domain.com and separate site to domain.com/help
- Login on Multiple Sub-Domains without Multisite
- Child Site doesn’t appear in the Network menu
- Update User Role Across Network when Main Site User is Updated
- Assign role to user on first login, if they are first (after admin)
- WordPress Multisite/Network – How to use a subdomain as main blog and subdomain for child sites
- Get the User ID Who Owns a Given Blog ID in Multisite
- Make a user administrator to a sub directory site and a contributor to main site in multisite network
- WPMU Development Environment
- turn single domain off
- Redirect subdomain in Multisite installation?
- Enable wildcard in a WordPress Multisite (Sub-Domains)
- Multiple wordpress installations that shares WordPress & Theme but with separated database
- WordPress Multisite Wildcard Subdomains on Plesk
- Multisite – One user allowed access to all sites?
- Making 2 WordPress sites mutually searchable and more
- WordPress Multisite Question
- Multisite – 404 when accessing sub-sites
- Migrating wordpress multisite and domain mapping plugin
- Unable to apply theme to WordPress Network Site install
- Four columns in the wp_users table
- multisite subdomain behind reverse proxy
- Multisite not using subdomains as hostname in from address when sending emails
- Multisite – Looking for ideas to best manage a main site change
- ERR_TOO_MANY_REDIRECTS when i config my subdomains
- Subsite access without being a member of the subsite in wp multisite network
- get post id by $content (so by post content)
- Multisite Subdirectory with same domain subdomain
- My subdomain on WordPress Multisite has gone
- Set up Network Locally on a Flash Drive
- How to display users with posts published between two dates (Sorted by Post-Count) [Multisite]
- How to show only blog-domain instead of the whole permalink in the loop
- Get users registered 30 days ago(or more)
- Copy user role on multisite so the user can access subsites with same role
- WordPress in subdirectory, Multisite using subdomains Nginx
- How To Disable Add new users On Subsites In Multisite?
- How to query blogs posts of WordPress sub domain?
- WordPress Multisites or Multiple WP Single sites?
- Wait ajax to complete before continue loop
- Multisite on single wordpress
- WordPress Multisite with subdomains on different domains
- How to delete user from MU site when the user is removed from their site?
- WordPress Multisite Add User
- Installing other applications alongside WordPress multisite and configuring dns routing
- Customize or avoid new admin email address notification
- How to share User Database between Two Multisite Installations + More
- can’t create user without email for an author after converting single site to multisite
- Restrict users of site 1 to login in site 2 in wordpress multisite
- “My Sites” incorrectly showing all network sites for all logged in users
- How can I have two or more WordPress Pro (paid) themes that form a single website?
- Easily adding multiple existing users to a multisite site
- How to convert multisite subdomain from http to https
- Installing other applications alongside WordPress multisite subdomains
- Possible to have duplicate usernames on different two multisites
- Multisite – each site with it’s own set of users
- Multisite – SMTP not working on other network sites
- Multisite – site user limited only for this site
- Multisite install on a sub-directory
- Single sign on to sub-sites in Multisite Network
- Importing Posts into New Website with Same User ID’s
- Using both subdirectory and subdomains in Multisite
- Is Multisite the RIGHT option for my case?
- Get current user outside of WordPress Multisite
- How to configure CNAMES to be part of a WordPress Network
- Multisite: Run a subdomain install from a directory install
- WordPress MU users – how are they organized?
- Pointing multiple subdomains at the same WordPress Installation
- Is it possible to put a site other than #1 as a Multisite top-level domain?
- Using a number for limiting registering or banning on multisite
- Unsure of my options, Multi-blog?
- WordPress Multisite (Network) some site users added to main site users list as subscriber
- Cookies in Multisite network where sites have their own domain name?
- Change a subsite Admin role of a WordPress Multisite after 24 hours registering