If you specify more than one attribute whose values should be considered for prefixing the format name for a given element <elem>, the order in which you list the attributes to be considered in [ElemSectionName] (see §6.5.2 Listing attributes whose values can affect output formats) determines the order in which the prefixes will be applied:
[ElemAttrPrefixes]
attrx = *
attry = *
If the default format name for <elem> is Elem, for each instance of <elem> that includes values for both @attrx and @attry, their values will be prefixed to the format name in the order the attributes are listed in this section (not the order they are listed in the element itself), and the name of the format DITA2Go looks for would be xvalueyvalueElem.