You'll likely be maintaining a regular uni workload of 4 subjects a semester regardless of it being a single degree or a double degree. The difference being that, say, a regular Science degree will have space for electives and such, which in a double degree is replaced by the core subjects of the other degree (education in this case), often the length of the degree will be extended a bit too (4 years in this case). So given that, it's not necessarily true that a double degree would be more hectic.
I'm not too well versed on studying education, but from what I'm aware of if you're going to study education after completing a Science degree, I would expect that to be taking something like a Master of Teaching or trying to get into the Teach for Australia program.
To take a Bachelor of Education afterwards would mean taking the full duration of that degree, on top of 3 years of your Science degree. It's a lot more streamlined to just take up the double degree. On the other hand, if you want to absolutely load up on the science subjects without any other areas getting in the way, then taking the Science and then a graduate level teaching program will probably be the better pathway.
Keep in mind that if you start a double degree, you can just drop one of the components too (alternative exit if you want to drop the education part,
http://monash.edu.au/pubs/handbooks/courses/1633.html bottom of that page) or transfer to the single degree. Again, I'd probably advise on looking into all the pathways of becoming a teacher since I don't know too much about it, but hopefully I've given you some perspectives to think about.