I have a master bath with a 78 inch long vanity and just one sink located at the right end of the vanity. The 2.0 waste line runs to the left inside the wall to a 4.0 dia pvc pipe about 85 inches away that goes to the basment to the septic and runs straight up thru the roof as a vent. I believe it vents the sink and toilet. I would like to tie in between the present sink and the 4.0 pvc for the second sink. Will I need a new vent pipe between the two sinks or will the one 4.0 vent pipe be ok?