Every attribute in metaobjects has a value "Descriptive", which takes an int as an argument. From the wiki:
This property is used to display attributes in the Reference Set Selector widget.
What you need to do is give each attribute that you'd like to use as descriptive a unique number. At the moment both of your attributes have Descriptive value '1', so to fix that you can either remove the descriptive property of one of the attributes or set it to '2', or '3', or whatever.