Is it possible to run more than one client instance on a machine? Is it possible to run more than one Bitcoin client at the same time on the same machine? I tried to set different working ports, but it seems not to be working.
You need both a different RPC port (-rpcport), a different P2P port (-port), and a different working directory (-datadir).  You will also need to use the daemon (bitcoind) for everything after the first instance, since only one GUI can be running at the same time.