if you have 2 different domains like siteA.com and siteB.com - This is more issue of CORS (cross origin resource sharing). It is related to the browser security and the way it works. If a user trying logins to siteA - you want to login him to siteA.com and siteB.com at the same time. From the browser prospective you issue authentication cookies for 2 domains - that normally won't work due to the single origin policy, but you can find a way around it by adding proper headers to your response/request.
Assuming you have web.config for siteA (although in you case it is probably shared web.config)
To make it work for siteB you should put something like this in your web.config :
<remove name="X-Powered-By" />
<add name="Access-Control-Allow-Headers" value="Origin, X-Requested-With, Content-Type, Accept" />
<add name="Access-Control-Allow-Methods" value="POST,GET,OPTIONS,PUT,DELETE" />
<add name="Access-Control-Allow-Origin" value="http://www.siteB.net" />
<add name="Access-Control-Allow-Credentials" value="true" />
These custom headers will instruct the browser that it can accept cookies for siteB.com but a user is on the siteA.com domain. There are some issue down the road: you can only one Access-Control-Allow-Origin in web.config. There are ways to solve it (google it, stackoverflow will save your day). Access-Control-Allow-Credentials - will let issue cookie for siteB. You probably do not need all the verbs in Access-Control-Allow-Methods. Anyway this will get you started.