[dexter1]

The Indigo2 riser board has three GIO64 connectors, of which the top two connectors are GIO1 and the bottom connector GIO0.
Putting the Elan in GIO0 and the XZ in GIO1 should in principle work: Go to the PROM and check 'hinv' , if both are detected, you're fine.

Both cards are Express graphics, so the IRIX OS should not need additional drivers or replacing the X.eoe subsystem. But you probably do need to reconfigure the kernel:

1)  ioconfig -f /hw
2)  autoconfig -f
3)  reboot system

if that doesn't help, probably a reinstall should be in order.

Jan-Jaap's post below raises an important point. Your PSU should be capable of running dual-head. What is the part number of the PSU?