I was lead climbing Number 2 gully in a group of three, we were at the last belay before the final pitch which required a gear-less traverse left under the cornice. This was approximately 20m from the top on a solid rock belay.

I noticed a group of three soloers were ascending the route below us, they stopped at the previous belay around 10m below, they called up and asked about top out conditions and cornice issues, We updated them from what we could see at the belay.

Upon hearing the condition report they soloed up in single file to the left side of the belay following the obvious line, and they took a moment to view the final pitch with a better vantage point. We all discussed route conditions then we encouraged the soloers to go past us as it was safer for everyone.

Soloer 1 climbed up 6m past the belay on good neve. Soloer 2 and 3 began to follow, soloer 2 reached maybe one or 2m higher than our belay, soloer 3 was around parallel to the belay.

At this point soloer 2 lost their footing and began to slide, both axes were placed picks into the neve, but as they were straight shafted axes their hands slid straight down (I believe the axes might have had wrist leashes but were unused).

Soloer 2 proceeded to slip into soloer 3 directly behind them. Soloer 3 plunged their axe shafts in to brace for impact, but unfortunately couldn't stop the slide. The collision caused both climbers to slide down the gully without axes or means to arrest.

The soloers slid the full length of the gully and ended up far down into coire na ciste out of sight, I would estimate at least 200 meters of falling down steep snow, probably being stopped by the reduced angle of the slope and avalanche debris that came down around them. The two casualties were separated by 40 meters when they stopped.

At the belay we had discussions with soloer 1 about their safety. They continued on the route as they felt it the safest option. We called mountain rescue who deployed a helicopter to rescue the two causalities.

Other members from my climbing club were nearby in Coire na ciste and were some of the first on scene. Soloer 3 had a dislocated shoulder but otherwise both soloer 2 and 3 were relatively unharmed.

The members from my club talked to the causalities and learnt that soloer 2 appears to have been their first ever winter mountain climb and they were on a university mountaineering club trip. I don't know the experience levels of soloer 1 or 3.