Goestudio (Slope/W) is the defacto slope stability program to use when dealing with slopes.
I wonder any geotechnical engineer would miss such a well designed tool ? The student's version is free with well documented manuals . Gstbl7 can not be compared to Geostudio .It just like comparing...
Your best bet is ASCE ,Geotech. Here is a good ref. :
_Ohori,K et. al. "Static analysis model for double pile wall structure" , Journ. of GEO. Engr. , ASCE, 114, No. GT-7,pp 810-823
Cheers,