It sounds like your trying to connect two different appliance types using a common chimney with this remodeling project.
Lets see if we can clear up some terms first:
I take it stack is what you are referring to as a chimney?
Is this chimney a masonry “brick” chimney? or a metal chimney?
Is the vent or venting by way of a metal flue pipe from the Water heater?
The basic concept is to exhaust the products of combustion properly and that means following requirements such as the appliance manufacture outline within the installation manuals that come with the appliance. To make changes / modifications that fall outside these requirement can lead to serious problems such as improper venting, back drafting or even worse Carbon Monoxide hazards. Poor chimney draw can also seriously affection performance of your appliance in certain cases so this is not something we can take lightly.
These science of proper venting is critical to a safe home so proper installation is first and foremost important and any changes to a vent system need to be performed by a professional who can properly configure venting.
Generally “Plastic” exhaust piping is used for high efficiency appliances (90 %+ efficiency, category 4) with positive pressure venting that require there own exhausts due to a higher pressure venting then the lower pressure (higher heat) natural draft (atmospheric) appliances like your water heater. In addition to this is the fact that the high efficiency exhaust have low temperatures that PVC or “plastic” can be use for the exhaust. So this means we cannot mix the two types of exhausts into a common chimney!
The “natural draft” or atmospheric draft system, lower efficiency (negative pressure) appliances with metal flues go to masonry ( hopefully lined) chimneys or metal chimneys that relies on the heat or “draw” created for venting. So what your aiming for is not recommended or advisable to do. I would look at the furnace documentation and contact a qualified heating contractor for assistance and information on what can be done to accommodate your remodeling plans. People have paid with their lives by not properly venting appliances…
Please play it safe…