I'd like to create a cluster with two different machine types.
How would I go about doing this? What documentation is available?
I assume you are talking about a Google Container Engine cluster.
You can have machines of different types by having more than one node pool.
If you are creating the cluster in the Console, start by creating it with one node pool and after it is created edit the cluster to add a second node pool with different instance configuration. This is necessary because the UI only allows one node pool at creation.