mmmm - I did a quick writeup, then figured it was so easy that it'd only take a minute to get the pics for you too.
Here you go:
First, set an Action Button event on map1 for event1, have whatever dialogue you want, then set a switch ON (not a self switch).
Then, create TWO pages for the event on the other map. The first page will be whatever happens if the player has NOT spoken to event1 on map1. The second page will be what happens if the player HAS spoken to event1 on map1. Use the same switch that you set in that original event, to condition the second event page.
Note, this will work whether the two events are on the same map or totally different maps.