How do I display unique symbology for two different fields in a shapefile layer?

[thrash]

I added shapefile data to a basemap which contains polygons of unique tree stands in a state forest.  There are a number of fields associated with each polygon.  I would like to display only one unique value from each of two different fields.  For example, I would like to display only polygons with the value of "SEED/SAPLING" in the "STAND DIAMETER" field as a unique color as well as polygons with the value of "ASPEN" in the "TREE TYPE" field as another color.  Most of these polygons will not overlap.  Using the "Symbolize your layer by category/Unique Values" I am able to display only the polygons with "SEED/SAPLING" value in the "STAND DIAMETER" field.  I cannot seem to find a way to display the "ASPEN" from the "TREE TYPE" field.  When I add a second field in the symbology pane, it combines values from the two different fields.  These two fields are not dependent on each other.  Can anyone help explain how to do this?  Thank you.

[DavidPike]

Is there a case when both fields will have a value?

I think an Arcade symbology expression would do the trick, or just make a copy of the layer Copy and paste features using the clipboard—ArcGIS Pro | Documentation, add a definition query Filter features with definition queries—ArcGIS Pro | Documentation to it, then assign the unique value symbology for one of the fields in each layer.

[SarahHartholt]

Which platform are you using- ArcGIS Desktop, ArcGIS Pro, ArcGIS Online? Using Desktop or Pro, right click on your feature layer and click properties then click on the Definition Query tab. Click on the Query Builder.. or New definition query button. Create a query that looks something like this (for ArcGIS Desktop):

STAND DIAMETER = "SEED/SAPLING" OR TREE TYPE = "ASPEN"

Your polygon layer should now only display features that contain these values. 

In the properties dialog box, click on the symbology pane & under show choose Categories -> Unique values, many fields. Using the Value Fields drop down, select STAND DIAMETER and TREE TYPE then click Add All Values. You can then create symbol groups by holding the ctrl key and clicking on values that you want display using the same symbology then right clicking and pressing group (ie SEED SAPLING, ASPEN; SEED SAPLING, BIRCH; SEED SAPLING, MAPLE)