Electrical – Activate two devices with a DPDT switch and two relays

I would recommend searching for buffers (or line drivers, transceivers) with tri-state functionality. Here is one example from NXP. They cost $0.57 USD each at DigiKey.

Basically, they act as simple buffers, but you can cause the outputs to become high-impedance and effectively disconnect the signals from your circuit. You would use two of them; one to connect (or disconnect) each of your devices. It is made easier because SPI lines, unlike I2C (for example), are each unidirectional.

There are hundreds of different ICs to choose from. For example, some have a single "Output Enable" (OE) line for all of the signals. Others have multiple OE's, allowing you to turn on/off only specfic signals. Some are inverting, some aren't, etc, etc. The architecture you need will depend on your specific circuit.

One thought: Perhaps you only need to switch your CS and WP lines? You may be able to leave the rest of the bus (including power) shared between both devices...

Good luck :)

You could combine multiple SSRs to do this. Two form B and two form A will give you a DPDT SSR. Cost would be fairly high (about $20 in singles) but this is really a weird set of requirements. Switching will be slow.

For example, two LCB710 and two CLA230 (IXYS) would handle 700mA.

You could also buy PV optoisolators and use them to drive back-to-back MOSFETs for each switch. Since the former come in pairs you'd end up with 8 parts (plus maybe 4 resistors). Again, slow switching.

Edit: Since you imply in the comment below that there is no requirement for power-off normally closed, the simplest method is probably to use two dual form A SSRs such as TLP222A-2.

Again you could use dual MOSFETs and PV drivers, but I don't see much point in that since the current requirements (and presumably your voltage requirements) are met by common parts.