There are many ways you can combine 2 maps. I'll go tell you the most basic one. 
 
Door = swing door used in level transfer.
Map01 = first map that links to second map.
Map02 = second map that links to first map.
void OnStart()
{
     SetEntityPlayerInteractCallback("Door", "Func01", false);
}
void Func01(string &in asEntity)
{
     FadeOut(3);
     FadeIn(6);
     ChangeMap("Map02", "PlayerStartArea_1", "", "");
}
Then on the other side of the door have it do the exact same thing except that have it be "PlayerStartArea_2" and "Map01" for the ChangeMap command function.
Hope this helps! 
