Yes, and if the Umax secondary processors are scarce as hens' teeth (which they are), the secondary processor in the Pios would be even scarcer (& needing connection with a similarly unfindable ribbon?). Most likely very few machines shipped at all, and fewer still with two 604ev cards in them.
On the other hand, the machine in question seems to me to be buckets more capable than the Umax clones, because presumably those can't accept a 604ev processor, in the same way that a 7500, say, cannot. There's a significant difference between the 604e and the 604ev, involving more than just 100 MHz. The whole chip was re-engineered.

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The cable connecting the two processor cards in the S900 was just a (?)14 pin ribbon cable. Easy enough to make your own even, if you have the crimp-on ends and the bulk cable. All readily available at Mouser and Digi-Key and their ilk. The scarce thing is processor cards wired for the ribbon cable, and secondary processor cards built in a mirror image of the primaries.

Still, I'm not convinced that that description guarantees that there was ever a secondary processor card built with a 604ev.

The differences in the 604ev processor card and regular processor cards are very minor. I consider it much more likely that PIOS built a modified 604EV card which will work in the standard PowerSurge (X500 & clones) slot than it is that Umax spun the S900 board for them. As far as I can tell, the only difference in the 604ev cards/slots and the older cards/slot are some minor power supply pin differences.