Since it is easy to calibrate any level you can read, thus confirm it's accuracy, the only question might be resolution.
If one audible level can resolve a smaller window of levelness, then use that. If two are the same then use either one. I suppose if neither of two are actually accurate and can't be corrected with an internal adjustment or shims, then one might try to average the readings but I think this is likely to lead to greater overall error. Of course the longer the level, the easier it is to read to any level's maximum resolution. Please understand the difference between accuracy and resolution! Accuracy refers to the reading being actually correct. Resolution only referrs to the size of the window of levelness that can be read out. You can have resolution to beat the band and if you don't check the accuracy it's meaningless. In electronics, practical electronics that is, most people say that 3 digits of resolution is almost always all that's usefull. Tom Fowle
