KB40099 (Ability to pass multiple selections from a selector to a prompted report or document) introduced how to pass multiple selections of ONE selector to answer a prompt. Sometimes, it's required to pass multiple selections from different selectors. To accomplish this, you need to add another parameter of &elementsPromptAnswers as seen in the example below:
Main.aspx?evt=4001&src=Main.aspx.4001&visMode=0&reportID=57E498A74FFB6EA1E5D63AADBE5D7CFC&reportviewMode=1
&elementsPromptAnswers={&Year@GUID};{&CurrentSelectionElementID:W45&}
&elementsPromptAnswers={&Category@GUID};{&CurrentSelectionElementID:W46&}
&promptanswermode=2