1. I don't think your proposed IP configuration is feasible. The reason is AXI CHIP2CHIP has FIFO inside the IP. If you instansiated two C2C IP, those two IPs will work independently. There is no guarantee that the C2C stream output into Aurora 64B66B TX input is aligned.
2. If the 256-bits data is not AXI protocol, why not using Aurora 64B/66B with 4 lanes instead ? You will have 256bit AXI-stream I/F on Aurora IP.