The biggest issue at a lot of single locks, especially going up hill, is a lot of lock tail bridges don't have towpaths through them, so involves bow hauling the butty back, which can be ok, until you need to swing the line under said bridge, and fight any weir streams. CRT in their infinite wisdom have also redesigned most of the split bridges as the original engineers got it wrong and left a gap, and the modern office bods deem that too dangerous, so bang goes the gaps for bowhauling lines.
It is entirely do able, but in certain circumstances it is tricky to achieve, and the age old argument of wasting water whilst stood next to a bywash with a good few inches flowing over is always the first line of reasoning as to why it shouldn't happen. If it is necessary and achievable i always allow the other boater through, but at times it can be safer to allow the pair through together.