Ive also looked at many roofs of 3 storey houses with multiple baths
one on every floor using the same stack and have instaled 4 basement
apts using the same stack as the 2 above, Ive not had a problem in 15
yrs on 4 conversions. Im no plumber but Ive " shared" stacks. You are
talking "Air" venting which should not " suck " anything out of a
different toilet . I ve also been on a few larger apt building roofs ,
I have only seen 1 stack per run. If that were the case then large
buildings of 100 - 500 toilets would have the roofs covered with stack
vents. Pouring 5 gallon of water down a vent won`t creat a suction,
maybe pouring 500 gallons in a few seconds will. I still say the 5000+
guy is wrong , unless it is a 1/4 " pipe which it surely is not. . It is
most likely 3" or 4 " venting.