I thought the problem for most tankless vent was the coolness of the exhaust creating a potential for flue condensation & corrosion, but those bottom-of-the line atmospheric-drafted Bosch units are low enough efficiency that with the designed-in dilution air makes 'em ventable with the cheap B-vent (provided the draft is sufficient for both appliances whether one or both were firing.)
Some may require double-wall B-vent, but only the higher end forced draft versions require Z-vent. But again, it's the LACK of intense heat and the relatively high dew point of the undiluted exhaust of the forced draft versions that's the problem, not the high heat intensity that's the problem.
Vented into improper stack types I'd be more concerned about carbon monoxide hazard from corrosion pinholes than fire.
What model was Piggy #2's?