The "Ta" sound is made with the tip of the tongue placed behind the front teeth cutting off the airflow and then rapidly moving down to open the path for the air to 'explode' out. The "Ka" sound is made with the middle of the tongue pressed against the roof of the mouth cutting off the airflow and then rapidly moving down to let the air 'explode' out (the tip is down while doing this). So, the tip of the tongue and the middle of the tongue set up a 'rocking motion' alternately cutting off and releasing the air. So, try this slowly until you can detect the cutoff/release in each position. Then, begin to build speed while maintaining the rhythm between the tip and middle. So, when you say you can't double tongue - do you mean you can't make the 'Ta' and 'Ka' sound at all or just can't do it rhythmically and even? It may be a bit like trying to rub your head and pat your tummy at the same time but repetition will allow it to become automatic - and then faster - after awhile.