I notice the HiFiBerry OS in the set up (so far is "experimental") let chain two Beocreate boards via toslink with just one Raspberry Pi. Having a stereo set up. The set up is quite simple, just need some patience. I use two upscaled B&O CX100 and the final result sounds great.
I find a pair of toslink female-female connectors (couplers) and a short toslink cable to fit inside the speaker, one inside each. I glued the connectors and connect the board as per instructions. Then a 6m tousling cable to connect the speakers. Each with his own power source.
I got the toslink cables and adaptors in Amazon.