Hi, welcome to GoY, there are over 200 species of Hibiscus, so we really need to know which species you have, and whether they're different varieties of the same species, are they in the same part of the garden, or in different parts, does 1 get more sun than the other, sorry for so many questions, but the information will help you to get a better answer, also some species are fully hardy, others are not so hardy, and best grown in a greenhouse, Derek.