The standard rule is that you should only put one plant in a pot. It has to do with overcrowding, root systems, competition, diseases, and things like that. I follow that rule for houseplants and any plant that will last for more than a season. However, if you're planting annuals then I say go for it if you add more than one. They're going to die anyway once they're spent. Just be sure to gauge how big they'll wind up once they mature fully. Overcrowding the plants will do them no favors. Cascading flowers look great in hanging baskets.