Frank, just because the furnace's inducer pressurizes the vent on pre or post purge doesn't make it a Cat. III or IV appliance so no, it is not positive vent pressure as far as the codes are concerned. ANSI Stds allow up to 45 seconds of spillage before it is termed "backdrafting". Still a serious problem but I wanted you to be able to keep the terminology and therefore, code references straight.

As Jerry pointed out, the smaller vent is expected to enter higher than the larger appliance. This is to allow more vent rise so it has built up a head of steam before it tries to enter the common vent.
HTH

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