I agree wholeheartedly, making packaging a part of the production process makes dividing workers as growers and deliverers effectively useless.

I just go this game and found this confusing and frustrating as well. The way I found to get around this limitation is to use corridors to separate the rooms.

Basically you need to separate the three steps you want to overlap to three different rooms. Here's an example for Cannabis if you want Farmers to Grow, Dry, & Cut and Delivery to Box, Store, & Load.

Growroom A: Planters, Driers
Growroom B: Cutters
Storeroom A: Boxers, Storage (connected to Garages)

Then go into Employee > Room settings, create the following groups, and assign them to your employees.
Farmers: Growroom A, Growroom B
Delivery Growroom B, Storeroom A

Farmers will take the product from the Drier into Growroom B, cut it, then leave it there. Delivery will take the cut product from Growroom B then bring it into Storeroom A to box and store. In task assignment, you just want your delivery guys to have the associated products(s) ticked on in addition to "Loading"

Something to keep in mind is that you also have to assign the Breakroom, Bathroom, Kitchen, and Bedroom to the Room Group or the employees won't use them.