Probably neither one is totally drafting correctly by itself but OK to get by until you fire the second unit at the same time.
I don't believe venting into a brick chimney is up to code if there isn't a liner installed. All of the water heater install info I've every seen cautions against venting into a brick chimney without it being upgraded to a liner system.
I see it all the time in old house and has usually worked with no problems but with two into the same vent pipe then into the chimney I could see it causing problems when both are on at the same time.
Here's the installation manual for your water heater:
What it says about venting, (Doesn't say not to vent into a chimney):
The connection from the water heater vent to the stack must be made as direct as
possible and of the same diameter as the vent outlet. The recommended slope of
any horizontal breaching is at least 1/2" rise per linear foot. A certified draft
regulator (barometric damper) shall be installed in the venting with a location at
least 18” downstream from the water heater. This water heater is designed for use
with type “L” venting.
The stack must extend at least (3) feet above the highest point of the
roof to insure proper venting. The stack should be provided with a
weather cap of approved design.
Note: Provisions shall be made to prevent contact of the vent pipe with
combustible materials in accordance with all codes and regulations.
A separate vent and barometric damper for each appliance is strongly
recommended, consult the National Fire Protection Standard For Oil Burning
Equipment NFPA No. 31 (latest edition) or CSA B139 (latest edition), for vent
sizing and installation information.
Draft reading in the stack should be -.02” W.C. to -.05” W.C. High draft
may be caused by over firing, or too much excess air. If there is back
draft caused by down draft, DO NOT operate the burner until this
situation is corrected. Back pressure (back draft or down draft) may
also be caused by the chimney termination being lower in elevation
than surrounding objects, such as buildings, hills, trees, rooftops, etc.
Back pressure may also be caused by an exhaust fan in the building.
Edited by Budget Appliance Repair, 03 January 2014 - 09:06 AM.