When splitting models layer by layer, users in r/LocalLLaMA have reported good results with as low as PCIE 3.0 x4 as the interconnect (4GB/s). For tensor parallelism, the interconnect requirements are higher but the upside can be faster speeds in accordance to number of GPUs split across (whereas layer by layer operated like a pipeline, so isn't necessarily faster than what a single GPU can provide, even if splitting across 8 GPUs).