I use Holborn everyday and actually, I think it could work once people understand it. Though it's been rather hit and miss this week so far.
Basically, in Holborn there are four escalators leading into a smallish hall with tunnels one side for the Piccadilly Line, and the other side for the Central Line. So exit, entrance and interchange all happens in this one hallway. In the morning, one escalator is going down, and three are going up.
During rush hour, it gets chaotic as lots of people are all squashed into this small hall, either trying to cross each other to change lines, or to get onto the escalators.
What they've been doing this week mainly involve the exit from the Central Line tunnels, which is right next to one of the escalators. So that escalator they are asking people to stand on both sides, to try and channel more people out of the Central Line tunnel quickly. There's still an option to slide past that and walk up one of the other two up escalators, which are mainly taking people from the Piccadilly Line.
So I do actually get what they are doing. And at least they are doing something, as Holborn has been insane the past few months --though if you ask me I'd make it exit and interchange only between 8.45-9.30.
Also, the downside to walking in Holborn is that they are very long escalators, and if you realise you're knackered there's no room to slide into the right and stop!