(while i think this might extend to any two-block tall blocks, ive only conclusively had it happen with the runewood obelisks)
placing runewood obelisks next to each other and breaking them with the spellweaving axe causes the whisps to break only half of any obelisk they find, and they get collected as full items. breaking the remaining half also gives you the full item, meaning you can duplicate them indefinitely.
(while i think this might extend to any two-block tall blocks, ive only conclusively had it happen with the runewood obelisks)
placing runewood obelisks next to each other and breaking them with the spellweaving axe causes the whisps to break only half of any obelisk they find, and they get collected as full items. breaking the remaining half also gives you the full item, meaning you can duplicate them indefinitely.