Very few folks ever used the dual CPU feature.
The secondary CPU card was a special card. You can't just move your old card to the secondary slot. This is mentioned in the articles you referenced.
Both the secondary card and the primary card needed a special cable header which was used to connect the two CPU cards directly. That's not mentioned. I was hoping I still had a photo of the Umax CPU card in my web space, but I don't. Anyway, the lack of that connector and whatever function it provides would prevent you from using dual G3s, as Umax never built any G3 upgrades with the necessary connector.
Software support was the same as for any other multi-CPU machine from that era, i.e. nearly non-existent except in a few applications especially written to take advantage of multiple CPUs. The OS takes little or no advantage of a secondary CPU. I'm not sure how it would work if you managed to get OSX on the machine. I think there was some limited support for multiple PPC604Es in Xpostfacto.
I think the bit about adjusting the CPU speeds was correct, but I think it was very limited, as in, both CPU cards must run at the same bus speed, but the CPUs on the cards could be set to different multipliers of the bus speed, yielding different CPU speeds. The primary card sets the bus speed. But it works better if they're both set to the same multiplier.
That last bit is a quality of how this family of Mac logic boards is built. On all of them, 7500 - 9600 and clones, the bus clock speed is set by the clock on the CPU card. The clock is run through a clock buffer chip on the CPU card and split into seven separate clock signals. One of those clocks runs to the CPU on the card. The other six leave the CPU card by pins 9 - 14 and supply various components on the logic board with the needed clock signal.
Anyway, the Umax multi-CPU cards were only made with either PPC604 or PPC604e chips on them. The header on the top of the card was standard dual row .1" pitch connectors for IDC cable. I think it used 14 or 20 pins. If one was using a primary card by itself, with no secondary card, then three jumpers were needed on the cable connector in place of the cable. And while the primary cards would work in a regular Mac, the cable connector on top of the card got in the way of closing the case in some models, because some of them had that little CPU-card-supporting slot that went over the top of the CPU card when one closed the case.
SmallDog sold a bunch of the Umax PPC604e/200MHz primary CPU cards at clearance prices and I bought ten of them, I think. That was back when a 200MHz 604e was still worth $100 or more. I ended up desoldering the cable header and putting in flush jumpers to avoid the lid fit problem in Apple machines.
Here's an image of the S900 logic board showing both CPU slots. It's actually two scanner images pasted together. That's why there's a vertical artifact down the center of the image. As you can see, the secondary slot uses exactly the same connector as the primary slot, and IIRC, it is wired *mostly* the same. I traced out the pinout once, as that secondary slot makes a very interesting form of PDS slot in which to install a system upgrade.Full size original