I assume you are speaking of obtaining residual shear strengths.....you can use 3 normal loads on the same sample. However, the direct shear test is messy for this procedure and a torsional ring shear is recommended (ASTM D6467). You can also check out Tim Stark's research in the Geotechnical...
This is originally an old post and don't know if the original poster will be monitoring it anymore but......pseudo static analysis is applying a static factor to slope stability to somehow pretend to simulate a transient load. Thus the seismic coeffiecient does not relate to the pga in any real...