Can I have a shared exhaust flue for a high efficiency furnace and tankless water heater?
In answering @Harper-ReinstateMonica's question in the comments, I stumbled across something on page 12 of the manual which states "never common vent this appliance with another appliance or use a vent which is used by a solid fuel appliance."

So it looks like it common venting with a tankless condensing water heater would be generally unsafe, is not recommended by the manufacturer, and may be against code.