I'm certain this is a ridiculous question. I'm either missing the answer
in the docs or it can't be done.

I've got dhcp clustered between 2 servers. I want to know if I can use the
same 2 machines to cluster an additional service.

While attempting to create an additional cluster with the 2 machines an
error informs me that the machines are already members of another cluster -
choose different servers. The error seems fairly straight forward; it
can't be done. Can someone confirm this for me? I was under the
impression one could cluster two services between two servers; each running
a different service, yet be the failover for the other.