second sink vent

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?

vent

Under our code you would have needed an auxiliary vent as soon as the pipe was 48" from the sink, much less 80"

Under the 2006 IPC a two inch pipe can run, with proper minimum slope, eight feet off a main vent stack. It must not have any rises in the pipe and must come off a T not a Y, but I believe it can only work, be code compliant, with a single fixture, so in your case, if I followed you correctly, it might be OK now if the sink is the only thing on that pipe, it is sized at 2" one size up as required, add a sink and no.

Section 906.1 and 906.2

Thanks to all.

Thanks to all for your replies.
Kingnus