A two-beam second-harmonic generation technique is developed to calibrate the magnitude of the second-order nonlinear optical susceptibility components of surface and bulk (multipolar origin) of isotropic materials. The values obtained for fused silica calibrated against ChiXXX of crystalline quartz are chi parallel parallel perpendicular = 7.9(4), chi perpendicular parallel parallel (+)gamma = 3.8(4), parallel perpendicular perpendicular perpendicular(+)gamma = 59(4), and delta' = 7.8(4) in units of 10(-22) m(2)/V. Similar values are obtained for BK7 glass. An alternative way of calibration against ChiXYZ of quartz is demonstrated. The technique could also be extended to characterize the susceptibility tensor of crystals as a convenient alternative to the Maker-fringe technique.