I have a similar situation where Husband has died and two daughters are joint and several attorneys for Wife (their mother). The property is unregistered and sale will now lead to first registration.
My thought is that one daughter acts as Mum’s Attorney and using your (very helpful) suggested wording she appoints her sister as the additional trustee? They are both PR’s for their Dad but I don’t think this really adds anything as legal title has passed to Mum.
Have you ever had any requisitions raised by the LR using this wording please?