I know you can define in your xml which groups that you want to run, but I want to know whether it is possible to say run these methods if they are both member of groups A &
You can do this with the TestNG beanshell support: http://testng.org/doc/documentation-main.html#beanshell (you are getting access to method, testngMethod, and groups)
In your particular case, it would be something like this:
You could also do a System.out.println() if you wanted to debug stuff.
The catch here is that you need to define the "method-selectors" in the "test" block. For some reason, if you define the "method-selectors" in "suite" (which is syntactically correct), the filter is not applied.
Debugging this with System.out.println() is going to help you understand whether the stuff even runs, or which methods it filters, so e.g.: