If you mean with "spanning" that a VM could allocate ressources from both servers than this is impossible with all 3 products.
There is VMware FT which "mirrors" the VM in realtime to the other host, but this is for BC reasons.
BTW, the old IRIX OS which runs on SGI Origin Servers was a single Image OS which was able to "span" over all connected ressources.
Regards
Joerg