How do I check for doneness?
Probably not as it's only 1" high. If it were and width/length but 2".high (like lasagna pan) yes bc that one has 15c volume and two 9" diam ones have total 12c volume. Check allrecipes or other sources for a cake pan volume or conversion chart. Not sure about bake time for doubled recipe...
That should have been "same width/length"
I don't think you 'd be happy with the results. You would be better served to make two 9-inch cakes.
doubling and tripling are much easier than one might think; you just have to be flexible about how thick/high the cake turns out. Keep the temp the same when you double any baking recipe; start checking it around the time of the original recipe and keep baking til you touch it and it springs back (and a toothpick comes out clean.)
Now, given all that, forget trying to use that jelly roll pan-toooo shallow. Use a 'half sheet pan' or keep to the 9" round pans or a larger round pan- 12 " likely, or a 9x13 rectangular baking pan( ~2" deep, like a lasagna or brownies pan.
concur with all.