How competent are you with both of the languages? If you have little to zero knowledge of both the languages and you're planning to learn them both at the beginner level in the HSC, I wouldn't recommend it. However, if you do have some knowledge of one of the languages, either by self-study, travel or through formal lessons, and are doing it at the Continuer or Extension level for the HSC, I would recommend it. Whilst French and Spanish are similar there are some marked differences. For example, in Spanish, I believe that you do not use the first person subject noun when speaking, contrary to French. In Spanish, the way you pronounce and conjugate the verb undertaking the action indicates what the subject is. So basically, in French you would say, 'Je suis désolé', which literally translates as, 'I am sorry', the 'je' translating as 'I' and being the subject pronoun. In Spanish however, it is 'lo siento', which literally translates as 'sorry about it', with the subject pronoun being non-existent, and the verb of 'sentir' being conjugated in the first person, which indicates that the person is speaking in the first person. Learning either one of the of the languages, however, will help you immensely in learning the other later on as they are from both the same language family (the Romance language family) and have many words with similar roots e.g. 'bien' basically means the same thing in both languages. Also, French and Spanish are one of the easiest languages for an English speaker to learn.
EDIT- just read your other post that indicates that you're an accelerated student in French. So, with that in mind, go for it!