This is the way most climbers go about hauling if it comes to it. The ‘sack is pulled up first on one rope (usually one half of your double rope system in the UK), then the second follows on the remaining single rope being belayed by the leader. This works fine on steep or compact ground, but on anything less, there are various problems of which to be aware of.
Firstly, if the ground between the ‘sack and the hauler is complex then there is a good chance that you’ll end up getting the ‘sack stuck which wastes time and is highly frustrating. The chances of this are reduced the more streamlined, lighter and well-suspended the ‘sack is but, nevertheless, in my experience, if there’s the chance it’ll get stuck it probably will. If the ‘sack gets stuck you’ll have to wait for your second to reach it - if that’s even feasible - in order to un-stick it.
Don’t be tempted to start yarding on it in order to dislodge it as this could send a block down on to your mate’s head. This highlights the main danger of this system, in that the ‘sack may dislodge rocks as it rises, which may well strike the second - or their rope (a single skinny rope). If the route the second must climb is dangerous (sharp edges, flakes or loose), then it may also not be healthy (physically or mentally) to go swinging around on a rope as thick as a shoelace. The two systems below are designed to overcome all these drawbacks.
This system is almost identical to the standard haul except the second does most of the hauling, rather than the leader, doing so as they climb. The second hauling when and wherever it is feasible to do so.
The set up is the same as with the standard haul, except that the second climbs up above the ‘sack then hauls it up after them, rather than the ‘sack going first. Once the ‘sack is raised to the second’s stance the leader above locks it off so the second can climb higher, repeating the operation until both second and ‘sack is at the belay. This set up is great for slabby or complex ground, or where the ‘sack is too heavy for the leader to haul, or when the leader is positioned in such a way that they cannot apply enough power (haul line running over lots of edges for example).
This system is perfect if the ground is dangerous and loose. It allows the second to remain attached to both ropes and reduces the likelihood of being hit by falling rocks dislodged by the ‘sack. A secondary benefit of this system is that it also allows the second to have more control over the ‘sack.
The second remains tied into both ropes, attaching the ‘sack a metre or so higher up one of the ropes, so the ‘sack remains out of the way, but can be reached and manoeuvred easily if it becomes stuck. The second can also manoeuvre the ‘sack from below using the rope to guide it - and if they believe it may knock down rocks - climb above it while still remaining tied in. This system requires good communication between both climbers and can prove hard work for the belayer if the rucksack is heavy. One option is to put the haul strand of the two ropes through some kind of self-locking hauling device and the free rope through a belay plate. If the ‘sack is light the best device is a magic plate (Petzl, Cassin, Kong, NewAlp) as this will allow the leader to have both ropes locked at all times (otherwise you need four hands). This system also works well if the second chooses to jumar up the rope, as this gives a back up in case the rope abrades over an edge.
Lastly, the best advice for ‘sack hauling is to make sure you’re strong enough and your ‘sack is light enough, that you don’t need to haul it in the first place.