Are your different applications on different domains (e.g. - www.myapp1.com, www.myapp2.com)? If yes, the cookie that is set by myapp1 cannot be read by myapp2. You can use the same login, but the cookie is specific to each domain.
On the other hand, if your apps on on different subdomains (e.g. app1.myapps.com, app2.myapps.com) you can manually set the cookie domain to .myapps.com so it will work across subdomains. If a user logs in on one application, it will work across the others as well.
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