I want a unit to follow another unit via a trigger, the thing is that I want this action to be executed only once, thus one order to let the unit follow another unit until it (or the other one) dies or it becomes impossible for the unit to follow, simply just like as if you would do a rightclick on a unit of yours with an other unit. Info: both are ground units and there is like no way unit one can escape the terrain where unit two is in.

modify move order's distance in data editor (or copy it and set it to 0) and try to get the move order onto the unit. I'm not sure if that is possible atm.

Don't make it harder than it has to be...

Follow
    Events
        Game - Map initialization
    Local Variables
        UNIT_TWO = Marine [25.41, 95.39] <Unit>
        UNIT_ONE = Ghost [22.95, 95.26] <Unit>
    Conditions
    Actions
        Unit - Order UNIT_ONE to ( Move targeting UNIT_TWO) (Replace Existing Orders)

You mean order targeting unit? Returning an error that "Move(0) needs a target."

Then you are doing something wrong, i.e. assigning a unit that doesn't exist.

I think having Conditions that detects if both units are alive would be smart, btw.

Hmm i think that sounds buggy...
What if unit drops follow, due to some zergling action or.. i dont know, like in my new map (just started) GTA they drop when i "almost" drive over them.. Anywhoo... I would create a region thats attacked to the region. And if unit LEAVES region.. that is the following unit, he gets the action again (to follow center of region) And if you would like to follow the unit, but alittle from a certain vertical or horizontal axle you could just move the offset or whatever of the region that is attached =)
GL HF (Im not a big fan of explaining with pictures and script, i do expect ppl to understand when doing something as complex as creating something, and not just playing it) -> Else tutorials tutorials and tutarials

"gion thats attacked to the region."
What i ment was region attached to the unit that is suppose to BE followed