Having a PCI POST card might cause the ISA POST card to not display anything. Often the ISA bridge uses "subtractive decoding" and forwards all cycles to the ISA bus that don't get taken by a PCI card. Assuming your PCI POST card claims the cycle, the cycle might not be forwarded to the ISA bus. I don't know whether it is possible to "passively snoop" port 80 on the PCI bus without claiming the cycle (i.e. asserting the DEVSEL line), or whether the data written to port 80 doesn't appear on the PCI A/D pins at all in that case, so I expect a PCI POST card would claim the cycle.

Most (not all) PCI boards you will not get an ISA Post card to show anything at all no matter what (just my experience). I doubt using two, one ISA and one PCI will give you any better results except on a rare few boards.
As mkarcher said, the output on port 80 may be only thru the PCI or if grabbed by PCI first will not show on ISA bus....