You can make InnoDB use two or more separate devices without using RAId or something, you just need to specify multiple files in your innodb_data_file_path variable
innodb_data_file_path = /ssd1/ibdata1:1024M;/ssd2/ibdata2:1024M;/ssd1/ibdata3:1024M;/ssd2/ibdata4:128M:autoextend
will put the first GB of data from your database on the first SSD, then the 2nd on the other, etc.
It is probably best to use the 'innodb_file_per_table' which makes a separate innodb file for each table, then you can just symlink different tables or databases to different drives, to load balance is however you think will give you the best performance where you need it.
There are both advantages and disadvantages to doing it this way. The best solution for speed is to have enough ram to keep all of the innodb data cached in memory as well.