I do Engn and have a few mates that do Engn/IT. And i know a few that used to do the double, but dropped the IT bit. The double degree is no harder than an uncombined BIT or BEngn, just takes an extra year. They both interlink somewhat, ie COMP1100 and COMP1110 are 1st year prerquisites for IT and ENGN. BSoftware Engineering interlinks even more.
I cant say anything about majors in IT, but the majors in engineering are basically split into mechanics or electronics, you wont be able to do a mechanics and an electronics major with a double degree, but it might be possible to do 2 electronics majors.
One of the best thing about the engineering degree is the university electives, basically you get to chose 6 courses from anywhere else in the university(ie arts, commerce, ect...), but these are sacrificed if you do a combined degree.
You dont really need to decide on a major untill the end of 2nd year. Engineering give you about 23hrs/week max(on a very bad week). Its a little lower if you throw some IT subjects in place of engineering subjects because you dont do any labs. But easy to have a casual job at the same time, unless you need HD average for a scholarship or something, then things become interesting,..well if youre in this category then you wont have any friends to spend money with anyway so it doesnt really matter, and since you'd have a scholaship you probably won't need a job. But dont dwell on that too much.