How to get the values on the other form
mymodule_admin() is not the function that does the trick saving your variables. It's system_settings_form(). 

You can retrieve variables saved by system_settings_form() using variable_get() function.
So, in your case, you can get the value like this:

$txtSubscription = variable_get('txtSubscription', '');


Second parameter of the variable_get function will be returned if the variable is not set (not necessarily empty).
Notice that we used the same key as your form array's key. 

During the Drupal bootrap, all variables are loaded to $GLOBALS (Thanks to kiamlaluno for the correction). Do not use this to store all your data because all of these variables will be loaded on every bootstrap.