I have two layers that contains 5 polygons each with their own values. I would like to make a global color ramp that encompasses both layers. But I am not sure how I can do this. I have tried setting the number of classes under gradient colors to 10 for each layer and create my own color ramp, but ArcGIS automatically resets this back to 5 as there are only 5 values for each layer. The images below display my two layers: 0.25 degrees and 0.5 degrees.
In cartography (and graphic design in general), not only color shade can be used to represent information. See this table from the excellent Jacques Bertin's Semiology of Graphics which lists the different visual variables and the way they should be used to represent information graphically.
For your case, you could consider representing one variable with color shade, and another one on top of it with a texture (like hatches). This can be done with ArcGIS easily.
You could also consider making a bivariate color ramp. See these articles, which describe more into detail the method: