But then this works exactly as I want it to do

Use train ticket no 1 day 1 AMS->Amsterdam Centraal
Use GVB ticket day 2 in Amsterdam, maybe 3 trips on busses and trams, activated in the morning day 2 on first bus or tram.
Use train ticket no 2 day 3 Amsterdam Centraal->AMS

The combination tickets costs 13.20, and I only have to stand in line once.

Regular price for individual tickets would be 3.70+3*2.60+3.70=15.20 or 3.70+7.00+3.70=14.40 if 24h card.

My fear was that the 24 hours would start ticking when entering the train at Schiphol noon day 1, and last only until noon day 2.