In the scenario where two teachers are grading through the rubric simultaneously, the only scores that will be saved are the ones most recently saved. The other scores from the other teacher will be overwritten and they will not show up in the grading history. According to Canvas Support, "this is due to each rubric being a copy of the original and not combining with each other. Additionally, if one teacher pulls up the rubric and enters their portion of the grades then save and then have the other teacher go in and do the same, both of the grades would show up in the grading history with the most current saved score showing in the gradebook. There wouldn't be a way to combine the scores/rubrics but you could see the grading history."
Simultaneous Rubric grading should be supported in Canvas, complete with grade change logging, or it should blocked so that another teacher cannot start a rubric grading session while another is in progress.