By vital I mean essential to life. Without the protein the cell will die. I'm not talking about complexity as in the NUMBER of proteins produced - I'm talking about having a system which incorporates this ability to code for two essential proteins on a single sequence of DNA.
In general only a single strand of the double stranded DNA codes for proteins - the sense strand. It's complementary strand, the antisense strand isn't used (for this you have to know that A on the sense strand is T on the antisense strand, C is G, G is C and T is A). This dual coding system is able to use the antisense strand to code for proteins as well as the sense strand, but not only that, it uses a part of the antisense strand that complements a coding region on the sense strand.
I'm emphasising this on a particular level to illustrate that complexity is a subjective quantity - it depends on who's doing the looking and what they're looking at.
Sorry about the error in the link, I gather you found it.