To stop the boat yawing about on a single anchor line to the main hull bow, set up a bridle that takes the weight of the anchor line to each of the ama bows with the line to the main hull slightly slack [but still cleated as back-up].
Most of the time this will work, but if not, shorten one of the bridles a bit to keep the boat pointing slighty one side of the wind, this is what the 2nd carbine is for [the original can stay cliped on as back-up]
You can also experiment with rudder angle, board depth and wing mast tying to help things settle down too.
A stern anchor just gives the boat 2 anchors to snatch against and would need resetting everytime the wind swung or the tide changed.

Paul

F32 #81