As from Help
Specifies the key of the column whose cell to position the special row prompts in.
You can specify just one column (by its Key). The only other option available (as far as I know) is to let the property to null. In this way the prompt spans from the first column over the adjacent columns.
Of course, your code, set, as prompt field, the column at Index 1 because the last one used overwrite the previous setting.