Duplicate Surface Error in Site XM

I am trying to open two fairly large (123MB and 143MB) dtms.  Originally there was supposed to be one huge dtm but because of the file size it was split into two.  I can open one but when I go to open the other I get the screen saying "Duplicate surface. Do you want to overwrite 'file name'?"  The surfaces are different, have different names, filenames, perimeters and descriptions so I can't understand why I can't open both of them.  I am trying to run a template through both and i dont want to have to keep switching between them.