Elastic scattering of x rays in 3D: the principle for XRD analysis
In 3D you will see spots not cones from a single crystal.  If you Google for "XRD spot pattern" images, you will probably find yourself enlightened about how many different shapes, and forms diffraction patterns can take.  

Debye-Scherrer rings are one special case which occurs when you have a diffraction pattern from a very fine powder.  In such a powder you are summing the diffraction patterns from thousands to millions of individual crystallites.  In this case, you can imagine a large number of spot patterns rotated randomly around the beam axis so each spot projects into a cone.  Each individual crystallite, however will produce a spot pattern, and there is never anything but a spot pattern from a single crystallite.  (Sometimes spots can be smeared when there is strain in the crystal, or when the crystal is too small to produce high quality diffraction, but these are really cases of imperfect crystals.) 

You can see diffractions from rays transmitted through the crystal.  However, X-rays tend to be absorbed within a few microns to millimeters in most materials so that limits the size of crystal you can use for transmission X-ray diffraction.  

Finally, I noticed you said "Two parallel and in-phase x-rays..."  I just want to note that diffraction occurs with a single photon.  A single photon can scatter, and produce constructive/destructive interference with ITSELF.  You need to do this with many photons to see a full interference pattern, however, and this is usually the case in experimental situations.  If this is new to you, then take a look at Young's double slit experiment.

Hopefully this clarifies things a bit.