I am on the 2006 IPC code.

Kitchen is on the first floor, full basement below. There is a 4" CI stack serving a bathroom running in one wall. On that same wall, I'd like to install the new sink plumbing, and nearby add a 2" floor drain for the kitchen. (This will be a normal 2" shower drain)

In the basement directly below, there is a bar sink which drains into a 2" ABS line that goes into a CI hub and disappears below the slab. About a foot away, there is a floor drain in the slab. I assume these are connected underneath, and then run to the 4" line coming from the bathroom? Venting on the bar sink is currently 1.5" ABS that is connected to an old galvanized 1.5" line above the highest fixture; I assume that pipe rejoins the nearby stack (which does go out the roof) at some point in the wall.

So:

* existing 2" floor drain - not sure where it runs to, if it is sharing a horizontal pipe run with the bar sink or is entirely separate back to the 4" horizontal line.

* existing 2" bar sink drain - I assume this used to be an old laundry standpipe or something, the trap on the sink is a normal 1.5" P-trap but then it's upsized to a 2" line afterwards and goes down through the floor.

* new 1.5" kitchen sink drain

* new 2" floor drain in kitchen

Can I run the two new drains on a 2" line and join the vertical 2" line from the bar sink? Or would it be better to tap into the 4" CI stack? (If I do need to tap into the stack, I'll be having a plumber do it - not about to touch an old self-supporting CI stack myself!)