I have been looking for this answer as well and I think it is Haiti and one other?
That was Haiti and Liechtenstein. The flags were the same with the top half blue and the bottom half red. After the 1936 Olympics, Liechtenstein added a crown to the upper left of their flag so there would be a difference.
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Liberia and Malaysia both have a flag with 13 alternating red and white stripes and a blue canton on the upper hoist side. Both of them even have a "star" in the canton (though Malaysia's is yellow rather than white and also features a crescent moon).
A little less like the US flag, but still pretty similar: Puerto Rico has a flag with 5 alternating red and white stripes and a blue triangle on the hoist side with a white star in it. Cuba's flag is identical to Puerto Rico's, except the stripes are blue and the triangle is red.