Don't have a specific sample holder or you always wished for two or more glass-slide sample space in your microscope holder, just take the path of 3D printing assisted rapid-prototyping and make one for yourself.
Please note that you might need to adapt the dimensions according to the stage you have. So please make this sure before using the design as it is. Also it is pretty easy to modify such designs to add-on spring loaded clips, magnetic clips to fix your sample :)

In the shared design: One is for 35 mm petridish holder
and another is a 2 glass-slide holder.