Sum values in xmlstarlet from two siblings?

I am using xmlstarlet to extract text from elements with a specific preceding sibling event. An example from the XML file:

11.134
    0.2
    CUER
  
  
    11.367
    1.26
    TARGETR
    379
    1
    YOU WIN!
  
  
    12.651
    2.65
    1
    YOU WIN!

From the example, I need to do the following: 1. print the onset of ` $DIR/output.stf ``` Though as written, the following code only displays the duration from the matched element rather than summing the duration of the two adjacent events. **Is the latter possible using xmlstarlet?** Thank you for your help!