You can just do :

/castsequence reset=900 Astral Recall, Hearthstone

This will cast Astral Recall on the first click, and Hearthstone on the 2nd, resetting back to Astral Recall after 900 seconds (15 minutes). If you want to do Hearthstone 1st then Astral Recall change the order and increase the reset to 3600 (60 minutes).
Keep in mind that the castsequence timer will reset itself if you relog.