Attached photo - We are looking from the floor below the bathroom being remodeled. The new bathtub is over our head and I am looking to run the drain line.

(C) is the existing p-trap for the shower with (A) being the vent up and drain down.

Two plumbers have told me they will tie the bathtub drain into the existing shower drain going through joist at (E) and tieing in behind the joist (D) and use existing vent/drain (A).

Shouldn't I run the bathtub drain through joist at (E) and run to (B) with a vent that will tie into the vent (A) halfway up the second floor wall and 90 to (A) for the drain? 1/4" slope per foot

Can I tie into drain at (A) with fitting one on top of the other or do they make a fitting that will tie both into (A) at the same level?

Sorry - The view is from the ceiling below the floor of the bathroom being remodeled.

Just can't see enough of the plumbing to really say for sure. If the job is being inspected, let the plumber do his thing and let the inspector sort it all out. If its not being inspected, draw up a diagram (if you can) and post it here so we can get a better idea of what we are dealing with.

Your plumbers are wanting to wet vent the tub through the shower vent, This is acceptable if wet vents are allowed in your area. Has permit been pulled and will this job be inspected? Back to you, Tom

Wet vents are allowed but they are proposing to tie into the shower drain during its horizontal run instead of connecting at point "A" (which is a vertical tee from the shower drain, vented properly up and draining straight down to the first floor).

I am wondering if I should/can tie in the new bathtub drain at "A", while having it vented up at point "B") The vent isn't a problem as it would tie into the shower vent half way up the wall on the level above.

If I vertically stacked two sanitary tee's at point "A", one would be a wet vent for the length of a tee?