Merging SD Card

I have just read an article about merging the SD Card. It is the first time I have heard of this possibility and would welcome further information or links. I am guessing I would need to backup the data already on the card, perhaps to an external drive, and then restore it afterwards? Are there any pros and cons I should consider?

My OS is Android 7.1 on a Samsung Tab A

from Android Authority Forums http://ift.tt/2G0iiuv
via IFTTT