Yes I was on reflection, but I wasn't thinking of the Lower Severn, I rarely do, and I am sure you are right they are very nomadic down here

In fact I reckon to target barbel seriously below the weir you would need a carp anglers approach and a week away from the missus.
But
I will stand by my point in a general sense that in most circumstances a two rod approach is risky, which I think has been demonstrated by even the most competent of anglers. I am thinking that this is a carry over from those that are either once or still are carp anglers, which is common practice on a still water, but a different set of circumstances with barbel on a River.