Fill both drains with water, have one person go to the other side and cover & hold the drain while the other person uses the air gun on the other side. I had two other people come 'fix it' and they didn't do anything.
Air gun huh? Sure glad I'm not 56the other person on the other lavatory when all that nasty water explodes out of the overflow, ( Did you forget about the overflow?)
Besides, ain't gonnba work anywat! The reason being that back to back lavatories are connected through a drainage cross with a vent coming off the top of the cross. The air won't even come close to clearing a cog in a back to back set up.
1. Because the clog will be found below and downstream from the cross and
2.Air guns only work when the clog's between the air gun and the vent. Otherwise all that air simply goes up the vent,
For back to back lavs there are only two ways to remover a blockage.
1. Snake from the lavatory roof vent, and or
2, Use a drop head auger tip, (see image)on the end of your snake to direct the cable downward to the clog.
Hope this clears things up about back to back lavatories. Cheers, Tom