I only know one good way to do this. It involves another control to deal with but is mostly automated. It’s a separate IK control for the extra leg bone and works like a pole target (but with greater influence on bone location).
My problema 2.blend (482 KB)
Best to just examine the file but here’s what I did (bone names underlined) :
Set the chain length on Lower bone’s IK constraint to 2.
Moved Pole target back close to High bone’s tail. This is going to be High bone’s IK target.
Put a Copy Location constraint on Pole target back targeting IK Bone. Settings are X Y and Z location toggled on. Y Inverted. Offset on. Local Space. Low influence. You can adjust influence and set Y loc inverted or not as you prefer (inverted Y seemed to work better with foot raised really high).
Btw Pole target back is not parented to anything. In a finished armature this and any other bone not parented to anything else would be normally be parented to a “Root” bone.
Put an IK constraint on High bone. Its target is High bone target. Chain Length 1.
I parented Pole target Front to IK bone so it moves with IK bone rotation. Some prefer it be parented to the armature’s root bone. Which way is best depends on specific needs and preference.
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