You can add a hyperlink which links to the second presentation.
To do so, first create a new folder.
Place both the presentations into the folder.
Open the first presentation, add a textbox or action button.
Right click on the textbox or action button, select Action settings.
Click Hyperlink to.
Click on the dropdown and select Other PowerPoint presentation.
Look for the file you want to hyperlink to.
Ok your way out.

Alternately, check out Awesome Background's tutorial for another method of
linking:
http://www.awesomebackgrounds.com/powerpointlinking.htm