U r unnecessarily getting confused between 3 things here... 1)Pps Expiry, 2)Medical Expiry & 2)PR Visa Expiry. All r unrelated. For 'landing' u have time till your PRV expires... that'll be related to the 'expiry' of the Pp u had sent for stamping; OR 1yr from the meds -whichever of the 2 is 'earlier'.
If the pp u had sent was expiring in 2 months -your visa will be expiring in 2 months. And thus, u 'must' Complete your 'Record of Landing' by 2 months only -irrespective of the 2nd passport's expiry date [which was obtained later -after the PRV was stamped].
However, if the pp u had sent for stamping was expiring later than 1yr -then your PR visa will be expiring on 1yr from the date of your meds. So, u'll have a 'longer' time to 'land'.
Hope that made some sense...
Qorax