for 2 or more terrains imported from GoogleMaps I always use the Clip or Trim Terrain Mesh plugin. No complaints. But of course, it was not made to use with two terrains that donĀ“t match.
But donĀ“t worry Sdmitch, I followed the "skirt" advice. While draping the terrain over the other terrain crashed Sketchup, draping the detailed terrain over a flat surface worked. Then I just deleted the border of the flat surface and I had my skirt.
(ps: just thought now that instead of draping, I could have selected the entire terrain, used the SELECT ONLY plugin, select only border edges... then the projection tools plugin to extrude lines along a vector... select the bottom edges and resize them... that would also create a skirt.