Mac Pro (6,1) Multiple VM's

I have vSphere esxi 6.0 installed on a 2013 Mac Pro hosting two Yosemite 10.10.4 vm's. The problem that Im having is that I can't run them both at the same time. If I do, they lose connection to the host and I have to reboot the server.

The VM's are only using 2GB Ram, 2 processors and 128MB Graphics memory.

Any Ideas?

Thanks