bathtub and shower on one p-trap?

[Pihls]

I am remodeling a bathroom to add a shower and replace the old tub. The existing p-trap for the tub does not line up with the tub drain as is conventionally shown. It is offset about 12 inches. My question is can I connect the new shower pan and the new bathtub to the same (existing) p-trap. The arrangement would be shower pan, and tub tee'ed into the p-trap, then vent, and then drain to the septic tank.
The structure is concrete slab. The water level in the p-trap is about 9 inches below the current slab floor surface so both fixtures would be above the p-trap water level.

[phishfood]

Don't have a code book real handy, but as I recall code does not allow for two fixtures to be drained into the same trap, with some exceptions.

[Reedwalker]

The vent needs to be between the fixtures as well.

Venting is the issue that forces inspectors to fail plumbers most often. Understanding venting methods and implementing them is difficult.

[majakdragon]

Doubt you can do this. Most Codes require a 1-1/2" drain for tubs and 2" for showers. Reason is that showers have low sides and the chance of flooding is more likely than with a tub/shower combination.