Nice idea. Thanks.
I realize a second one would be a little larger than most mugs, but:
1) nothing fits inside such a bowl as space-efficiently as another of the same kind,
2) why not make a pint of tea / whatever? We're always working to stay hydrated,
3) you'd have back-up for your bowl/water scoop,
4) when not used as a second bowl/cup (after dinner hot drink?), nesting the two bowls would make it a lot more insulated against your hands.
I hadn't found the perfect bowls for family trips (need 4 bowls, 4 cups) and this would be SO light and SO compact when stacked compared to other options. Cheap, too.