DL180 G5 does not boot after installing second PCI card

[Christian Hausleitner]

We are using an HP ProLiant DL180 G5 with Riser card option (2 PCI-X Slots) and want to install two PCI bus digitizers (CompuScope 12400) from Gage Applied Technologies.
Installing only one PCI card in any of the two slots works fine (PC boots normally) but when installing the second card (of the same type), the computer does not even boot. A blinking cursor appears on the upper left of the monitor and the System Health LED is flashing red.

PC Data:
HP ProLiant DL180 G5, Riser Option 2, 16GB RAM, 72 GB Raid 1 serial scsi, OS: Windows Server 2003 SP2

PCI interface of the cards:
32 bit, 66 MHz (or 33 MHz), PCI V.2.2 compliant, PCI V.2.1 compliant if 3.3 V, Throughput: up to 200 MB/s (dependent on motherboard and number of PCI-PCI bridges), Scatter-Gather: fully supported, Bux Mastering: fully supported, Plug&Play: fully supported

Any help would be very welcome.
Thank You!