A relatively new philosophy of root canal cleaning and shaping called Self Adjusting File (SAF) shows great ability to clean long oval canals. The problem still remains in apical third when SAF has a limitation to clean properly.
In my view the simple and probably effective way is to complete the apical shape by rotary or hand file instrumentation.
This tooth #27 (15) was endodontically treated due to irreversible pulpitis. The anatomy was like that- two merged buccal canals and a huge palatal one. All roots looked fused.
WL 24mm was confirmed by EAL after primary extirpation with H-file #20. Instrumentation was done by SAF #2 to working length. Apical preparation was completed by Nitiflex hand file #45 only.