Because I make comic panels constantly, I have a script that does basically what you are asking, but it requires some tweaking afterwards. I am sure the script could be modified to have resizable columns or rows, but for now, it's limited because it is a comic panel script. For example, your 8 columns and two rows, was easy for me to input, but the rows are equal in size. Who is to say you can't create two panels, quickly, and just merge them into one document, in seconds.
First row: 8 equals columns (took me 5 seconds in total to make)
Script found here:
http://registry.gimp.org/node/24632?des ... node/24632 (still works in 2.8.18)
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"In order to attain the impossible, one must attempt the absurd."
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