Double degree = 2 degrees done simultaneously e.g. Monash BSci/CompSci and is 4 years (4 units/ semester) vs. Monash BSci and then Monash BCompSci after, which would take 6 years (each of those single degrees = 3 years). This means you’re usually dealing with two faculties of the same uni and doing a least one major (main speciality) in each, and will receive two certificates at the end, each with a major minimum.
Pros: explore two completely different areas, more flexibility than double major, can do a major and minor (lesser speciality) in each degree
Cons: longer time-wise, costs more
Double major = 2 majors in one degree and is just 3 years full time e.g. Monash BSci double majoring in physics and comp sci. They are two majors in the same faculty/ similar areas. You’ll only get one certificate with two majors written on it at graduation.
Pros: shorter time/ less study, cheaper degree
Neutral: more specialised
Cons: little wiggle room for other interests (less electives/ minors possible)
Note: if you’re doing a double degree, it’s not more stressful than a single degree. It’s just you’ll be answering to two faculties vs. one and studying more units across two areas.