Given that there was difficulty in communication between the two cockpits. I wonder if confusion arose as to who was doing what. I recall and accident many years ago, also in a biplane - possibly a Tiger. Both survived, when after a go around each pilot thought the other was flying - a take over not properly heard. . It was said to be the only accident they knew of where both pilots had their arms folded !
That seems as likely as the 'incapacitation' option. Two experienced pilots involved- perhaps the owner started the go around then handed back control but that was not heard. Or maybe the owner felt the slots locking and the light control follow through and concluded the other chap was again flying her.
A very sad event - which rather reinforces my feeling, from personal experience, that the unintended 'underarousal' effect of flying with another known experienced pilot can reduce safety, ironically.