distance(user1.x , user1.y , user2.x , user2.y)
if character not mirrored distance(user1.x , user1.y , user2.x , user2.y)less then 200 trigger follow ( move towards angle 2 pixel)
if character mirrored distance(user1.x , user1.y , user2.x , user2.y)less or equal 200 trigger follow ( move towards angle 2 pixel)
repeat the code for both of them depends if you want them to follow each other or not!
if user1.x = user2.x or user2.x-20 move forward towards user2.x +200 then trigger follow again ! to pass through and then come back is a bit tricky, the whole process
—
the thing is you can also use LineOfsight that does basically the same thing what i said above! as 
Shadowdawnz said! but then you will need a trigger event or switch like when they face each other to be turned on but move forward then if distance is bigger or out of sight start following each other the thing is the whole process will make both characters wabble around each other at some point !
how ever line of sight is a bit buggy and may not work all the time, my fish game template even though i could use line of sight, i dident was creating this weird comparisons and then wouldn't attack my hero fish! so i used the normal distance (x1,y1,x2,y2)