the way I understand it, with the 2006 upc you can but prior it is not allowed.Here is a another wet vent setup, Min shower drain is 2" min vent for that shower is 1-1/2", using a wye combo to pick up the vent for the shower, increase that vent to 2", you can now tie a lav into that vent of the shower if the sink is next to the shower.
Dave2006 illustrated doesn't cover the new bathroom wet venting only the regular wet venting. its just hard for me to understand. But the good news is the inspector passed it. GC said inspector was on site 5 min made reference to my wet vent and gone.
appendix "L" page L-4 thank you I was right and it passed good deal. Wish that or a reference to it was in the rest of the book, would have made me more confident.Dave
Check the appendix. I'm not sure where, because I gave my copy to one of my apprentices, so I'm dealing from memory.