This assumes you are in your home directory - if not, type cd ~ before starting.
First create the chain in the default directory (~/.multichain/) in the normal way:
multichain-util create chaintest
multichaind chaintest -daemon
For this example let's say the node address output was chaintest@12.34.56.78:9255
Now create the second node as follows, using a different datadir, port and rpcport:
mkdir .multichain-other
multichaind -datadir=.multichain-other -port=10255 -rpcport=10254 chaintest@12.34.56.78:9255
You will see the usual message about needing connect permissions, so assign these via the regular node:
multichain-cli chaintest grant [address-shown] connect
Now you can start up the second node again:
multichaind -datadir=.multichain-other -port=10255 -rpcport=10254 chaintest -daemon
You can send commands to the second node as follows:
multichain-cli -datadir=.multichain-other -rpcport=10254 chaintest
To make shorter command lines in future you can add the following lines to the configuration file .multichain-other/chaintest/multichain.conf for the second chain:
port=10255
rpcport=10254
Then in future you can just start up this chain using:
multichaind -datadir=.multichain-other chaintest -daemon
And access its command line using:
multichain-cli -datadir=.multichain-other chaintest