It would probably just cause problems.

It can work in theory beacuse the "coding" part of DNA is just really 1 of the strands, the second strand is just there to make copying and error preventing etc easier. But adding more moving parts just makes errors that much more likely.

It would be more intersting to have more base pairs then to have more helixes. 

Having more then 1 coding strand would cause problems because both strands would have to code for something non-detrimental at the same time, which is unlikely to work out, or you would need to evolve 2 seperate transcode mechanisms (1 different one per strand). Having 3+ helixes also would remove the incredible space saving self folding up DNA can do, which is bad for error prevention.

edit: i did find this on wikipedia tho http://en.wikipedia.org/wiki/G-quadruplex might be a fun read.

Quoting DARCA1213,

reply 4

What about six, same number as carbon, would that be a sweet spot of stability? (come on all you intellectuals out there I'm to lazy to google it!)

 

DARCA

I suppose a civilization more advanced then ours could engineer something like that, including the mechanisms needed for repair and transcoding etc,  but the only similarities it would have with DNA is that it would encode genetic information.

I also don't know what the point would be. Human DNA has 64 possible codons which only code for 20 animo acids and a few stop codons, so you already have more capacity then you need with just 4 base pairs and 3 bases per codon. You would have to more then double the amount of animo acids your organism uses before you would have any real need for either more bases or base triplets.