AFAIK OOTB you can't start multiple instances of WSO2 ESB (or any Carbon based products) from a single installation as now.
But you can simply have multiple server installations (just unzip to wso2esb-x.x.x.zip to multiple folders). And run the server with -DportOffset option as follows,
$sh <CARBON_HOME>/bin/wso2server.sh -DportOffset=1
(Or use bat file if you are in windows)
Or if you want persist the port offset, you can open <CARBON_HOME>/repository/conf/carbon.xml
file then search for tag <Offset>
and set a port offset as follows,
<Offset>1</Offset>
And run the server with as usual (without the port offset option),
$sh <CARBON_HOME>/bin/wso2server.sh
HTH,
DarRay