I was sceptical of the second pair of gates when the lock was built, but its one of those things where the decision making process had an impact on the final design. A decision was made that the lock would only be 57 feet long "to save water" - given that Salterhebble locks, 3 miles away, are only 57 feet long there was a degree of logic, but protests were made and it was agreed that a second pair of gates would be added to allow 70 foot long boats to reach Sowerby Bridge basin. Had a 70 foot long lock been proposed in the first place I doubt anyone would have campaigned for a second set of gates to save water. Further, by adding the second gates, any cost savings were completely lost as the full length chamber had to be built.
Bath Deep Lock was arguably underthought - basically because no one expected much traffic and there was some official resentment at having to build it at all - for a while "fill it in" campaigns had competed with restoration campaigns, and even as it was being built (mid 70s) it was by no means certain Devizes locks would ever reopen. By contrast, Tuel Lane was overthought, certainly from a water demand perspective as the traffic forecasts were, in hindsight, a tad optimistic. BDL has a ridiculously short pound above it, but it copes even thoughbit gets very busy, Tuel Lane by contrast has a longish pound above it, the extra water for the 70 foot lock would be lost in the noise given the usage levels.