Split model / components

Having issues with splitting and export of model to stl file for use in simplify3d. I am just trying to create 2 1 in x 1 in boxes , export stl and then be able to use the simplify3d to infill 1 box at 100 % and the other at 10%. I can do this if the 2 boxes are not touching in sketchup pro when I do the export to stl. If the 2 boxes are touching in sketchUp when I export to stl and then use stlsimplify3d, mesh, separate connected surfaces I get one 1 x 1 x 2 in box and a second thin square ( what would be the middle of the 2 boxes). Any assistane would be greatly appreciated.