Multisites Single Sign In for domains solved this problem in no time! The package you download includes a detailed PDF file with all the instructions needed to get the plugin to work in less than 5 minutes.
I've installed RSFirewall (the 5th RSJoomla extension on my site, by the way) only this morning, and had two critical issues that were solved in no time (less than an hour) by the support team! They are not only incredibly fast, but also very efficient! Highly recommended.