You press the macro once for mark of the wild. Then press it again for thorns. It still requires two button presses, but you should see the tooltip change to thorns after the first button press.
I tried it just now with the following and it works for me:
#showtooltip
/cast Blessing of Wisdom, Blessing of Might
Also make sure that the macro icon is the big question mark, first option in the list. Otherwise it won’t show the proper tooltip.