2nd point - 1st point provides you with the directional vector 1st → 2nd. If you add this one to your 1st vector you will again get your second vector.
To get the middle point you have to do this ((2nd - 1st) / 2 + 1st). 2nd - 1st to get the vector between those two vectors / 2 to get the middle and + 1st to add it to the location of your first vector. Otherwise it will just start at 0, 0, 0 and therefore give a wrong result.
You might want to take a look at these:
Vector math is everything but UE related even though it’s very important for quite a large amount of stuff you want to do. Kahn academy has quite a few pretty good videos about different vector operations and you can even train it on their website!
Cheers! 