Ergo shifters are great, but they give no tactile feedback to what gear you are in. Sometimes it's hard for the captain to visually check the gearing, and stokers cannot always be relied upon. Even on a single, I have accidentally cross-chained while riding at night at the end of a long day, which is also when you get tired and the mental capacity diminishes. Because of these, I much prefer bar-end shifters on tandems and long distance bikes. I would be conservative and make sure all gears are safe on a tandem.