Forget practice, Camron. This is a theoretical discussion.
Let's take it as a given that the ball is knocked into the backcourt while under the control of Team A by A2 and B3 simultaneously touching the ball, and then recovered by A4.
The debate is to treat this as both being the last to touch or absolving A2 of being the last to touch due to the participation of B3.
By pure physics I believe that treating it as A2 being the last to touch has great merit. There is no denying that he touched the ball and that no one touched it after him.
However, from the standpoint of the written rules, I don't believe that the words were drafted with this context in mind. Rather I think that the rule was written to cover only a singular final touch. The implication of the word "last" in our language is a singularity. As in being the last to do something or to finish last. If there is a tie, it is usually specified.
Furthermore, the absence of words to the effect of "or simultaneously with an opponent" lends credibility to deciding this is not a violation. I just don't believe that the rules writers intended to penalize a team in such a case. However, I do hate hanging my hat on "the purpose and intent of the rules" though.