To get more Google juice to the problem. If you are trying qemu or libvirt or kvm or virt-manager and when you are starting up your second guest you see a message such like this:
QEMU quit during console startup
Then two of your guests have the same port set for their VNC devices. Change the port in the VNC section to something that is free. A more specific erro would have save a half an hour of Googling.