Ok, upon seeing my previous suggestion in type "2 keyholes and 3 or 4 Rams" might be a little dense (sorry), but this guy really seems like he wants to try two pair in a 29g.
Jezah, you have heard others advize not keeping more than one pair. I think it could be possible with lots of cover and the right pair (they all have different personalities, even within species), but I strongly advise you to have a second tank (it could even be a 5g bucket with a heater and fully cycled filter) ready to hold the second pair in case there are problems down the road, and be ready to return or give them away to a pet store or fellow hobbiest. If either pair does breed you will need a second tank to grow up the fry anyway.
I must mention that I am not as experienced as the people who advise against this, but I think with the proper preparations (second tank) there is nothing wrong with at least trying to make it work.