Multiple hard drives

Guys,

I currently have a hard drive plugged straight into my router. It acts as my movie library but is on the verge of getting full. My router has the capability for another USB drive to be connected.

I may use this to add a further drive to expand storage – question is – when KODI scans and access my drives, will they all be viewed as 1 library? How exactly do I tell KODI to merge these 2 sources as 1 library.

Many thanks for any help.

[Yes I am backing up all my media]