Can a jury overturn a verdict of first degree murder where the accused and another person conspired to kill the accused?

In Harbottle v. The Queen (1993), 1993 CanLII 71 (SCC), 84 C.C.C. (3d) 1, the accused and another person sexually assaulted the deceased and then, having decided to kill her, the accused held her legs while the other person strangled her. The trial judge instructed the jury that the accused could be convicted of first degree murder either on the basis of planning and deliberation under s. 231(2) or for causing death while committing one of the offences listed in s. 231(5). The trial verdict was first degree murder. On appeal, the accused conceded that he was a party to murder but argued that he did not actually cause the death of the accused. Because it could not be said with certainty on what ground the jury found him guilty, it was submitted that he should be granted a new trial. He argued that on a Smithers test for second degree murder he might have been convicted of that offence.