Indeed it is, and using the Spectranet board. The Pi simply provided synchronisation, each Spectrum had the whole piece in memory as BASIC code.

We had a bit of a discussion about how it might have been done back in the day using 1 bit input ports and a Spectrum doling out sync pulses.

Edit: Though the Spectranet's auto-loading of the code made things far easier than it would have been with loading 12 Spectrums from tape!