Side venting a water heater and boiler

A recently had a power-venting boiler installed (vents through a 3 or 4" pipe out of the rim joist) and will soon be installing a power-venting water heater. I'm wondering if there is an arrangement by which I can have the water heater share the last bit of the boiler's vent run with some sort of wye-fitting (thus eliminating the need for another messy hole in the wall of this 1912, balloon framed house with concrete between the fireblocking and the rim). If there's a way to do this safely and legally, I'd love to know about it.

Thanks,
Andrew Karre

From what I've read, the systems have safety features that look at the pressure difference between the intake and exhaust. The inlet/outlet of multiple units also have very specific manufacturer's specifications about how close they can be together, distance vertically and horizontally from windows/doors/overhangs and especially distance to dryer vents or sources of chlorine.

I'm not a pro, but do not think you can share either the inlet or outlet for your direct vented appliances and you must keep them separated as dictated by the manufacturer to prevent one from sucking in the combustion products of the other.