Multi-instance 2-node SQL Cluster - do instances need their own disks?
When I install a second instance, do I HAVE to use a separate set of disks, or can it be installed on the same disks as the default instance?


Yes you need to have a separate set of disks for the new instance.


  I'm not even sure if it is possible, let alone whether it is preferable) Am planning to host BOTH instances on a single node


That is not a bad idea, lot of people do and we call it as multi-instance cluster. Both the instances would be on same node in your case. They can also be on different nodes but you need to look at licensing part then.


  My concern is that one of the instances 'could' fail over to the second node. But the second node is not licenced, so am not allowed to run an instance on each node


In your scenario you have 2 instances on same node, this is fine since 2nd node is purely passive. BUT if a failover happens for "an" instance on 2nd node which is not licensed then you would have an instance running both on 2nd node and 1st node, this is a license violation if you run this scenario for long time, unless you immediately move it back such that both instances are on same node. The term "immediately" here means as soon as possible, MS is not going to charge you if let us say for hour you run 2 different instances on 2 different nodes.

PS: I am not a licensing expert but you should get this verified from local licensing expert of your region.