Hi Joe,
So what you are wanting to do is symbolize two types of features, ones that have values in a certain field that is greater than zero and ones that have values less than zero? Does that sound right?
If so, you could just add a column to your attribute table and give your features a value like "<0" or ">0" depending on what the real value is. Once you've got loaded into your ArcGIS Online map you could base your symbols on that column. You'd have two available unique values (<0 and >0)...