Unfortunately you cannot vent your furnace and water heater using just one PVC vent pipe. Each one of those appliances requires its own vent. Even assuming that your water heater will be a high efficiency / condensing type, just like your furnace, their common venting is not permitted.
If the chimney is 100% yours, there are no other appliances discharging into it, you may be able to run two PVC pipes inside the flue. However, there might be issues with termination of the pipes and fitting both of them (or even 3 if there are combustion air restrictions inside the condo unit) into the chimney flue
You probably need an experienced HVAC contractor to evaluate your situation.
One other option would be using an electric water heater instead of a gas fueled unit. Just remember that if your building has a common chimney (used by more than just your appliances), a PVC vent pipe cannot be inserted into it.
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