Hey Glen.. I would do a single card but for the fact that the Nikon scanner has a SCSI port that's got little clips at the ends of the slot, and the cable's got buttons to release from the catch - similar to the ADC connector - you squeeze it to release. That is the adaptec 2930 card. The other card, the 2906 is the straight up connector with screw posts. Different connectors, so no, I can't daisy chain them, I certainly do wish I could. I suppose I should just bite the bullet and install both cards and see if the machine can deal.
For whatever reason, the OS9 disk I have, which came from a friend who had a dual 1.25 MDD wouldn't load, It had the correct Mac OS ROM and everything, I think the CD was a bit too scratchy, so I managed to find a site online where they have a full disk image of OS 9.2.2, ROM and all plus EVERY updated driver - for the nVidia cards, and the ATI all the extensions and control panels.. everything but the last QuickTime. That's is what my OS 9 is nw and it's great!
Yeah, I've still got the living room media server, the wooden stand anyhow. The G3 power supply went up in a puff of blue smoke and it finally died. I'd swapped the original PS out with one from a beige tower and the power harness wasnt quite the same - IIRC I ended up with a pair of extra wires which I couldn't figure out were they went to so I clipped them off. That ran for 18 months or so and I ended up putting a Digital Audio G4 with a swapped-up 733 CPU into it. It was a snap, since the metal frame includes the backside PCI card rack. I found a new home for the 1Ghz Sonnet card with a friend that is, of all things, moving into the apartment upstairs, and it may yet be returning to my posession. It's living in a Blue & White G3 now though he seldom uses that anymore since I gifted him with a 15" 1.67Ghz PowerBook. (I had the fantastic luck to become good friends with the manager/level 2 service tech of a local Apple retail shop - he's been throwing me old kit and I've been just fixing and turning over machines that he doesn't want to deal with. Old kit.. better than filling a landfill.)
A few years ago, one of the posters here send me a nearly mint G3 complete with the super-duper bordeaux AV card and DVD-ROM and a pair of G4/400 ZIFs. That is in my computer-parts trunk and I haul it out when I'm feeling nostalgic and play old games on it. I *still* can't make a decent showing in Oregon Trail. I'm slowly building my library of classic apps up, as the storage volume on the MacPro where I kept them all, got borked when I installed Snow Leopard - it stripped the data forks off of a lot of the un-compressed apps and rendered a few inoperable. Even with FileTyper I couldn't resurrect them. Boo!
I was doing lots of scanning for a photographer friend and it was at times a project that threatened the living room. I pulled the SCSI card from that machine.. I still run it for the music since it's connected to the stereo system. Only change is an addition of a 720p TV, which, with the ATI Radeon 9800 Pro runs standard def videos just fine. They look pretty good too. So as it stands there's seven computers on the house network, and two rebuilt iPhones.
I'm definitely nuts at this point with the computers (menopause! nobody told me you could stay awake 50 hours at a clip and be wide AWAKE!!!! the whole time and have world-class 'speedy-brain' and just tear into everything out of boredom...) and am also going for the Big Dive, and get an Apple Tech certification. Why not, right?
Well, I'll put that second SCSI card in and see if it goes or not.
Wish me luck.
The answer to the main question is Yes, you can have two SCSI cards running at the same time.
But there are (were?) adapters to convert one flavour of scsi to another.
Here's link that could give you a start identifying what you have and therefore what you might look for:
"Well, I'll put that second SCSI card in and see if it goes or not."
As Eustace confirmed, you can have (2) SCSI cards installed concurrently. I had the 2930CU and a 2906 installed in a Blue & White G3. In terms of SCSI adapters, I have a few that convert the HD-50 interface to a DB-25, so they are available. Iomega's SCSI Zip drives had cabling with DB-25 connectors, so the adapter enabled me to connect to the 2930CU card. Most of my external SCSI enclosures had a pair of Centronics-50 connectors on the rear, so I used good quality cables with the Centronics-50 connector on one end and the DB-25 on the other, for connection to the beige Macs' onboard SCSI port. When my newer Macs lacked onboard SCSI, I bought the adapters to continue using the same peripherals with the 2930CU's HD-50 interface. Beyond the speed differential between the two Adaptec cards, an equally important difference is that the 2930CU is firmware-based, while the 2906 is driver-based. This means that the 2930CU is bootable, while the 2906 is not. If you only had (1) available PCI slot, the 2930CU and the adapter would be the preferred choice. I only used active terminators (not the less-expensive "passive" type) on all SCSI peripherals, and never had termination problems.