My original reply got cut-off, here is the complete reply:

I located the issue.. We are using Blue Tango’s AuthIntegrator module to establish a single sign-on for the base DNN parent (e.g., abc.com) and a custom sub domain site (subsite.abc.com). The required configuration changes for this service requires to include domain=".abc.com" in the <forms> attribute within the <authentication> section of the web.config.  This is preventing authentication to successfully authenticate the second DNN parent portal (e.g., xyz.com).  My solution is to split the two domains into separate DNN installs.