Two Sandbox VM's have two separate instances Ambari Server, as well as two separate instances of each of the master services. To add hosts to a Sandbox VM, I would recommend taking only one Sandbox VM, and then provisioning a clean, new VM with only Centos 6.7 (the current OS shipped under the Sandbox VM) on it.
After provisioning the new VM, you will want to follow the steps under Prepare the Environment with the new VM. Personally, I prefer to skip the first two steps of this (setting up password-less SSH and setting up service user accounts) and follow the steps for enabling NTP on the cluster, checking DNS and NSCD, configuring iptables, disabling SELinux and PackageKit, and checking the Umask value. Because I normally skip the step on setting up password-less SSH, if you do the same, you'll need to follow the steps to install the ambari agent manually on the new VM. When configuring the new Ambari agent on the new VM, be sure to set the ambari server hostname to be the hostname of the Sandbox VM.
Since networking can often be a challenge when doing this on your own laptop, I find installing the ambari agent manually on the new VM to be a great way to validate that I've correctly prepared the environment. Once you've done that, from your Sandbox VM, you can follow the steps for Adding Hosts to a Cluster to add the new VM as an additional slave and/or client node to your cluster using Ambari.
