Mirroring, Bodies won't merge.

Hey Guys,

This here is a body created by 3 sweeps. I was hoping to mirror the body and merge it to combine a solid piece (duh). The left image is when I use the mirror feature and select "Merge Result", the right image is when that is deselected. I tried to combine the two bodies post mirroring but has the same issue. Can anyone help me here?

SolidworksParts And Features