The only way to do that is to back up the data, create the JBOD and then restore the data - to switch from any disk configuration to JBOD (or vice versa) requires the disks to be formatted, which will destroy the data.
Also - if you value your data - I would strongly suggest you NOT consider using JBOD or RAID0 (which is the other disk configuration that will let you see the total capacity of the two drives as a single volume), if either disk were to fail, you would lose the data.
Last but not least - since you're asking a how to question and ending it without losing data - I suspect you have no backup for the data - not a very wise approach.