When I enter a bulk amount for the two years combined in Medical expenses section, the Medical expenses claim section defaults to 12 months. How do I claim for 24 months?

You cannot claim Medical expense for greater than a 12-month window, which you specify.

For 2017, the window must end on any date in 2017' ie it could include expenses incurred in both 2016 and 2017, but only 12- months worth.

The medical credit is so structured in order to allow that an expense could occur at any time, and expenses could be spread over one year to the next.

Obviously the same expenses should only be claimed once, regardless of what time window is chosen.