A while ago there was a discussion on twitter about a scenario which was talked about at VMworld, unfortunately there was a miscommunication and what was shown at VMworld as supported actually is not supported out of the box, so what are we talking about here? The scenario is one where you have two 2-node clusters (typically used for ROBO) and for each 2-node cluster the required Witness VM is running on the other. Let me show you what that looks like to make it clear:
The Witness VM on Cluster A is the witness for Cluster B, and the Witness VM on Cluster B is the witness for Cluster A. As it stands today this is not a supported configuration out of the box. For ongoing support, it is required that users go through the RPQ process so VMware can validate the design. Please contact your VMware representative or VMware Global Support Services for more details.
<Update: July 11th 2017> It has been decided that this configuration can only be supported for 2 node configurations by filing an RPQ, and will not be supported for stretched clusters.</Update>
