Hi Ptbronco...

Is this a two pipe direct vent that also pulls makeup air into the unit(s)? If so, you will need to read the instructions provided with each unit as each appliance has different requirements/clearances.

Even if these vents are only for exhaust I would still tell you to look at the instructions for each. Here, each appliance has very special requirements and we can't generalize... as much as I wish we could... ;)

Sorry I can't do better here, but with gas appliances there is just no room for error!. *UGH*

Mark