The bottleneck is the PCI Bus: it runs with 33 MHz and transfers 32 bit, so the theoretical maximum is 33 MHz x 4 = 132 MByte per sec. 28 MHz x 32 bit were seen only once with a special PC using very fast memory, the maximum total count rate with a P7888 on a standard PC is 20 MHz corresponding to 80 MByte per sec. You see that there is no way to increase the total rate by using more cards, as long as they are plugged into the same PCI bus, they have to share the total transfer rate of 20 MHz.
The P7889 is faster, as it uses the PCI-X bus; here the maximum countrate is 22 MHz, the datarate is 22 MHz x 64 bit, i.e. 176 MByte per sec.