This is an old request that keeps coming up in the forums. I believe the developer of zeroshell has already stated that the simultaneous-use param does not work in this build of zeroshell.
The quickest fix is to just recompile freeradius with the snmp switch to enable that feature. The other option is to setup a sql database and allow zeroshell to use it by uncommenting the proper lines in the *.conf files. Both will work.
A third options is to proxy the radius request to another radius server that can handle the simultaneous-use param.