This was tried in a number of stores in Sweden. It lasted a couple of months after which they reverted to one queue per cashier. Turns out it wasn’t popular, either with the vendor, the customers, or both.
On a personal note I found the practice irritating for not really clear reasons. One might be that it’s a chore to traverse the whole queue cordoning when it happens to be empty. In any case, over time being the one skipping the line when a new register opens averages out. Also, there’s the lucky draw feeling when you get to skip that otherwise gets lost.
A number of stores here in Kitchener (Canada) use the single queue approach, particularly to feed a bank of self checkout stations. But I've also seen it at Old Navy (clothing), Winners (discount clothing), Indigo (bookstore), and others.
I think the main thing that's maybe a bit annoying is just that it's easy for the single line to stall if the people at the front of it aren't paying attention or don't know what to look for. So the store often ends up having to have an extra attendant who just stands there and pokes people for when a till comes available.
On a personal note I found the practice irritating for not really clear reasons. One might be that it’s a chore to traverse the whole queue cordoning when it happens to be empty. In any case, over time being the one skipping the line when a new register opens averages out. Also, there’s the lucky draw feeling when you get to skip that otherwise gets lost.