I have created two indentical Websites. I want to use the same User. Ok, so far so good. If i registered a new User the System say:

Registrierung abgeschlossen!
Deine Registrierung erfordert Freischaltung und E-Mail-Rückbestätigung. Bitte folge den Anweisungen in der E-Mail, die du erhalten hast. Nach der Freischaltung kannst du dich anmelden.

Fatal error: Cannot use object of type stdClass as array in E:\xampp\htdocs\paytv\administrator\components\com_frontenduseraccess\plugin_user\plugin_user.php on line 34

The first say: Registered is done. You got a Email. Do what the Email said and all is ok.

But the Profile of the User is only read in this Site, where i registered it. On the other Frontend i can't login.

What do i wrong? Sorry my english is not the best.

If you want to share the users between website, this is something decided when you create the slave site.
See the tutorial video 16 and cross-check that all the MySQL tables relative to the user, core_acl and session are shared (link symbol) in the JMS tools / table.
www.jms2win.com/tutorial#jms12x-16

Can you tell me which is the extension com_frontenduseraccess ?