In reply to euanryan: twin ropes are more specialised, pretty much exclusively ice climbing really where you want lightweight, long abseils and two ropes just in a case one gets gets cut and the line is fairly straight up.
Twin ropes are designed to be clipped through the same carabiner each time, never separate. For all intents and purposes they are 'one' rope.
Half ropes sound like what you need, they're a bit thicker and more durable than twins and are designed to be clipped into separate pieces of gear. Nice and stretchy and tested to 55kg on the UiAA test (as opposed to single ropes 80kg I think). Giving you full length abseils, two rope strands just in case and all the other stuff re rope drag.
Neither doubled single ropes or half ropes *should* be clipped into one carabiner when leading. They flatten out too much which leads to surprisingly high loads cross loading the carabiner even when in the optimum position. You do see people doing this, and even clipping into alternative pieces and then through a single piece (ropes then rub against each other in a fall) but like most things in climbing think about it and make a decision.
Triple ropes like the beal joker can be used as all three types of rope, great for versatility but they would be relatively heavy half ropes compared to the rest of the market and very heavy twins.