@Dawn White
Dawn, I have the opposite belief - "Simultaneous" has a bad connotation (where you would use C's funds to pay AB closing) and the proper term is "Double Closing" (where you would use separate funds).
Terms aside, the proper way to do this these days is AB is funded by B's funds or B's borrowed funds and BC closing is funded by C's funds/borrowed funds.
As far as it being illegal, I am not an attorney, but as far as I know, it is technically NOT illegal to use C's funds to close AB. Before everyone stands up and yells... I remember seeing those letters from First American and Chicago Title around summer of 2010 or 2011 (I forget which year, long time ago) mandating all their associated title agents to ensure that both parties came in with separate funds. Therefore, It was just a mandate from the title insurers and title agents had to comply in order to be in good standing with their insurers in order to mitigate the risk of fraud as there was rampant fraud with "simultaneous" closings. Point is, you won't go to jail for it if you do a simultaneous closing, it just won't be ALLOWED anymore from that point forward by the big insurers/all insurers.
I have not kept up with this topic in a while, so please, correct me if I'm wrong.