You actually need two force plates in order to have the ground reaction forces of two legs in double support. Then, you can calculate the whole mechanics chain.
As a caveat to the above: force plates will allow you to solve for external forces. If you are interested in the internal shear forces acting between the sacrum and L5 you would need to use a musculoskeletal simulation to account for forces generated by muscles, ligaments, bones, etc.
See for example: https://simtk-confluence.stanford.edu:8443/display/OpenSim/Welcome+to+OpenSim