use the area2D body_entered signal, when both players are inside the area set two vars to true when both are true change scene
var player_1_inside_area = false
var player_2_inside_area = false
if player_1_inside_area and player_2_inside_area:
player_1_inside_area = true
player_2_inside_area = true
and i think you could also use setget to check for the vars value change instead of checking them on process every frame.