You should only use one tracking plugin, not both. In order to use the "Asynchronus" plugin in order to track the thank you page as well please try:

- disabling the RSForm!Pro - Google Analytics plugin;
- edit the form you want to track the submissions for;
- go to the "PHP scripts" tab;
- in the "Scripts called after form has been processed" you will have a variable '$u', this will control the URL of your thank you page. In order to make sure that the URL of the form page and the URL of your thank you page are different you could try adding an extra parameter to this variable, for example:

$u .= '&page=thankyou';

What this will do, if you have a form:

http://your_site_name/index.php?option=com_rsform&formId=13

when the form will be submitted the URL will be 

http://your_site_name/index.php?option=com_rsform&formId=13&page=thankyou

If you are using SEF URLs and you do not have any extra parameters in your URL then you will need to use '?' instead of '&' since the page parameter will be the first parameter in your URL.

This way you will have two different URLs for your form and thank you page and you can use the asynchronus plugin in order to track both pages.