I had to make a few more tweaks. Because the initial search is doing stats on each set of results, the output of the search (and thus the join) is not a "stream" but a stats table. Despite my belief that my columns were already multi-values, they were not (apparently) and I had to split them. Because it was table, streamstats didn't work. A simple switch to "stats" did the trick,
So the final answer is:
... | eval tmp = mvappend(split(colA, " "), split(colB, " "))
| stats count by key,tmp
| eval result=if(count=1,tmp,"")
| fields - count tmp
mvcombine result
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