Can I place ND filters back-to-back and increase optical density?

Yes, ND (neutral density) filters exhibit an additive relationship with their optical density values. For instance, stacking filters with Optical Density values of 0.6 and 0.9 yield a resultant optical density of 1.5 (which gives an overall transmission of 3%) Care should be taken when using the reflective type of ND filter in order to insure that the reflected light does not interfere with the application. In stacks, the reflective filters are not parallel in order to reduce back reflections.