Adding a 2nd tub to existing shower/tub stack

[TexasHighlander]

I have a 1st floor tub/shower on cement slab that uses an 1 1/2 vent pipe. I’m adding a tub only directly above it on the 2nd floor. Can I use the 1 1/2” vent pipe (exterior wall) for the tub only drain/vent? The existing pipe would be about 18” from the nw tub drain/p-trap.

 

[Terry]

If you drain the upstairs using the first floor vent for the tub, it will siphon the trap and allow sewer gas into your home.