Hi.
The Installer asks for the destination of the software when you start your installation. After all software is installed you are able to create a database (or later using dbca), and you are asked for destination paths again. Just give different paths representing your differnet disks.
It is usual to use more than 1 disk for a database: redologs seperated from "normal" datafiles, archivelogs can be muliplexed to 10 different locations, controlfiles should be on at least 3 different disks,... Reasons are performance and safety.
Stefan