I got served an ad for the Be Up belay device and while reading through its marketing blurb I found the following:
The unique element of the BE UP is its ability to allow the auto-locking and independent belaying of two Seconds. Effectively, whilst one free-hanging Second is locked off, the other can be brought up, or lowered, completely independently and safely. Also works when leading with two ropes.
While belaying the leader on a multipitch route using two ropes, the BE UP lets you pay out one rope while the other is locked-off, Be Up is the only such device that lets you do this;
The unlocking of the rope and lowering of one free-hanging second, without difficulty, with twin or half Ø ≥ 7.3 mm ropes. BE UP is the only such device that lets you do this, simply by inserting a normal HMS carabiner in the unlocking hole;
Am I being dim? Reading the manual, it seems that it is referring to putting a knot on one dead rope while guide mode lowering. Surely that can be done with any guide device? Also not sure what it is talking about when belaying the leader with two ropes either, as it sounds either like regular half-rope belaying or like locking off one rope by tying it off. I just can't help but think that I must be missing some obvious and important point here.