In a nutshell:
Offer the course of one degree, an entire year is essentially just electives. If you cut out all of those electives, then you can do two degrees in 4 years as opposed to 6.
You still study 4 units a semester, just like any other student, but half of your units are from degree A and the other from degree B.
There is some variations depending on the degrees in question, but the university website will let you know what those are (eg, most double degrees with engineering are 5 years, not 4, with an extra year of just engineering subjects, so the mix is more like 40:60 instead of 50:50)
Employers will never look down on you for doing a double degree - just because you earned your two degrees as a double, doesn't mean they're any "lesser" - you still have two degrees, and this is usually looked at more favorably.
EDIT: beaten by Jamon, left for the technical explanation.
