If you are doing joint preparation of returns with your partner,Turbotax will apply Medical expense to the partner who gets more tax credit for it.

Generally you should place Medical expense credit on the partner with lower Net Income, which Turbotax is doing . The reason for this is the 3% Net Income or minimum amount threshold which is applied to Medical expense non-refundable credit.

You can only determine credit distribution by avoiding the joint preparation process. Start new returns for you and your partner and in each case ensure you are doing individual preparation.

Be careful what you assign though, as you and your partner might pay more tax than you need.

CRA won't bother to change that!