If the filename is the same for both databases, you'll need to make sure the autostarted server names differ.
It is recommended to specify a server name if you are rebuilding a v9 or earlier database when the target database is not already running:
If the original database was created using version 9 or earlier of SQL Anywhere and the new database is not already running, you must provide a database server name in the -ac option. For example:
dbunload -c "UID=DBA;PWD=sql;DBF=c:\mydata.db" -ac "UID=DBA;PWD=sql;DBF=c:\mynewdata.db;Server=newserver"
In the command you provided, add a ServerName parameter that differs from the file name for the v9 database.