Onehourphotonic: It's schematic, but in reality it's one chip (with light source not integrated, I agree).

This one chip has two 'modes'. When memory mode is on, the RISC-V processor is inactive.

I think it is one chip design, but actually two chips in the testing setup.

To have Globalfoundries produce two different chips would have doubled the already high costs, so I believe they came up with a single chip design that allows a chip to be operated in one of two different modes: either in processor mode or in memory mode.

For the testing setup, I think they have taken two of these identical chips and told one of them to run in processor mode. This could have been done by some external circuitry, e.g. by applying some voltage to a certain "configuration pin" of the chip. Then they brought the other chip into memory mode, e.g. by grounding that configuration pin. Doing so, they got effectively to different chips which they connected by the optical wiring and – ha! – the external laser.

Admitted, I am just guessing, but that seems to be a plausible scenario to me.