How do I link two mirrored sides? So I've created an X and I used a mirror to make the right side of the X and have it symmetrical to the left side. As shown in the picture: 
The problem I am having is when the image is rendered it looks like this:

At the axis of symmetry for the X there is a weird black line? I think it's due to two lines meeting at the same points? I don't know how to fix this and I would really appreciate any help. This is my first time using blender ._.
Actually, this can be many things: 


First try to change the merge limit value, and have a render.


If it doesn't work :


in Edit mode, while zooming at maximum, make the twins vertices (of the top of your object, for example) -one for each side or your mirrored object - the closest as possible, overlapping themselves. 
Try a Render, it should solve the pb.


If not : 


if you don't plan to have further modification on your shape, once it's finished, in Edit mode, you can apply the mirror modifier, then press A (it will select all the vertices), then W, then remove doubles, it will merge the overlapped vertices
And try a render :)