What could be cause of a toothache that resulted an endodontist to perform two root canal treatments (due to deep cavities) and still there is throbbi?

Infection: Cavities may result in your teeth being infected. Even after the root canal treatment is finished, you may have some traces of infection and/or post operative pain. Please talk to your dentist.

Post op Pain: Good day; Post op endodontic pain may be due to the procedure or a need to have the bite adjusted; However, you need to contact endodontist ASAP.

You need to go back!: Call your dentist and they will either call you back in or give you prescription if they haven't already. You may have to go back in and have more treatment.

Be patient: This may last 7-10 days. If the throbbing pain does not eventually resolve, this may indicate a root canal complication and visit to your endodontist is warranted.

Restore too high etc: If you are still having issues after the endo was done then I would look at the current filling done (are they too high in the bite), secondly I would look at location (if upper it could be sinus related too) and third I would eval for possible fractures. Please have your dentist eval the area