a "cross tee" is not allowed, nor is a "double combo", for the two lavatories. It has to be a "back to back fixture fitting", and even then good practice would be to install a cleanout in the line below it. I didn't think the IPC did any testing. Their normal procedure would be if someone wanted to save some money he asked if the "easier way" could be done and it was approved then added to their "code", which was one reason their contractors rejected the agreement to merge with the UPC, since it would not be done that way afterwards.