I'll be interested to see if there is a working example of mc-lag to mc-lag connections. I tried to get this work in lab setups a few years back unsuccessfully.
The only configuration examples I could find all had mc-lag connected to standard ae ports on the other side. These showed connecting two mc-lag on one set of chassis to individual ae bundles on the other two chassis. So I assumed connecting mc-lag to mc-lag directly was at least not recommended but could not find anything definitive.
Your other option here is to put the spine nodes into a virtual chassis as a pair. Once in a VC your would be able to link the two VC spine using a normal ae bundle with a physical port on each switch.