In one case it was asked if an 11m antenna could be made to work on 6m with a tuner. The short answer is yes. The tuner will force the antenna to accept power from the transmitter and the antenna will radiate but how well it will radiate is another question. Again in short, the result will not be great but it will be better than nothing at all.
In the other case it was suggested that perhaps the 11m antenna be resized for 6m. In this case the antenna is no longer an 11m antenna and in fact has become a 6m antenna and will work well as long as care is taken to use the proper dimensions. Years ago I salvaged my old Wilson Shooting Star and used the aluminum to make a six element yagi for 6m. It works great,better than the comparable from Cushcraft. In this case I
am NOT using an 11m antenna on 6m but rather CONVERTED an 11m antenna into a 6m antenna.