Allamanda and Mandevilla are two different types of plants--same family but have never been synonyms of each other that I'm aware of. The other name that you'll find for some Mandevilla species is Dipladenia, maybe that's what you're thinking of?
I grew Allamanda 'Cherries Jubilee' a few years ago and it's a bit too upright to work well in a hanging basket--if you look at the pics in Plant Files you'll see that it's mostly an upright grower. Depending on the conditions it's grown in, it might flop a bit but I don't think it would be an ideal hanging basket candidate. http://davesgarden.com/guides/pf/showimage/214322/
Most likely what you're seeing is people using allamanda or mandevilla as a common name to refer to a plant of the other genus. As far as the Latin names go, Allamanda and Mandevilla are different genera and as far as I know have never been considered part of the same genus (at least not any time recently).