It looks like you're creating a cuboid which is 0.6 in size so it will have +0.3 and -0.3 X,Y,Z values for the first box as the center point of the cuboid is 0,0,0.
The second box is the same but offset by 0.3 in one axis which gives 0.3-0.3=0.0 and +0.3+0.3 = 0.6 values in one of the axes.
This will give two boxes that overlap half way on one axis which seems to match your picture.
Perhaps you mean to create a box with width/height/depth of 0.3/2 and translate it by 0.3/2 so that the center of the box is at 0.15,0.15 and use a scale factor of 0.15 instead of 0.3?