It appears that I may have a good solution for the problem of two SD cards on a single interface. The QS3VH257PAG8 is a 4 x 2:1 bidirectional bus multiplexer. It essentially is an on-off switch for signals going in either direction, and this particular chip's configuration has two switches in parallel with one side wired to a common pin and configured so that a selector input choses one or the other. Since this chip is a 4x, it would take 2 chips to handle the 6 pins on an SD card. Each chip also has an absolute enable, which we can tie to the GPIO pin from the controller that turns the card power on. The selector lines will go to another GPIO pin that will act as an A/!B line. When the select and enable lines are stable, the maximum propagation delay through the chip is 200 ps, which is mighty small (25 MHz is 40 ns), so I have fairly high hopes it won't get in the way.

Edit:

I've designed an experimental board with the v2.0 circuit (without the RNG) and one of these chips in place of the shared SPI bus. The SD initialization code will need a bit of adjustment, as the two separate !CS lines are replaced by an A/!B pin and a shared !CS line. But if it works, it will give me some measure of confidence that this will be an acceptable way to mux the cards on the next variant once the design with the SAME70 is ready. See? Knocking away the question marks a few at a time instead of all at once.