All I want is a check if there are any A/Values present in the B node set.

yes, but in relation to your "one line" question, the answer depends on
what you want to do. You can do that check with a single XPath, but if
you need to generate element nodes then since you can't generate nodes
with core Xpath functions then you need to write it as xsl, which
tends to take more lines, as you have to close the elements.