Missing the point of the qualities inherent to glass:

I see this a lot at school and it amuses me.  Since glass is transparent, another way to do this would be to stack both signs together and affix them on just one side of the glass.  By doing so, you get the following advantages:

• the flier facing the elements is protected by the glass, yet still readable
• four pieces of tape hold up two fliers vs. eight pieces of tape holding up two fliers
• you need not spend time carefully aligning two posters so they match each other