So you can't undertake two degrees simultaneously?
not that i've heard of. double degrees are different to what you're suggesting. in a single degree, you do four subjects a semester, all in the area that your degree is in (so in a science degree, you do four science subjects a semester). in a double degree, you again do four subjects, but this time what you'll normally have is two subjects per discipline (so in a science/commerce degree, for example, you'd take two science subjects and two commerce subjects every semester).
what you're suggesting is enrolling in two full degrees - that is, taking eight subjects a semester. this would most definitely kill you...
on the other hand, it might be possible to enrol in each degree doing part-time study - that is, two subjects a semester in each. many mature age students do part-time study, but it's quite unusual for school-leavers.