This probably isn't the answer, but hopefully you can run with it.
Something along the lines of..
sourcetype=sourcetypeA
| stats values(*) as * by publishId
| join publishId type=left
[| search sourcetypeB
| spath
| (Do the field manipulation/extraction you want here, might have to use rex, eval, foreach or a combination of the 3)
| table fieldyoucareabout, publishId
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