A promising approach for direct detection of quantum entanglement in solid state materials is probing with entangled particles. X-rays are ideal for characterization of many materials but have not been generated as entangled pairs in useful numbers, even at the most powerful synchrotron sources. This proposal describes how x-rays in fully entangled N00N states (with N = 2) can be produced by applying Hong-Ou-Mandel interferometry at an x-ray synchrotron source. The technical requirements for indistinguishable photons, x-ray beamsplitters, and interferometer design are met with existing resources. The entangled x-ray biphotons can be filtered from the unavoidable single photon background by simple diffraction from an analyzer crystal, resulting in intense beams suitable for characterizing quantum materials.