As far as I know, the --settings option supports just one settings file. Worse, when it is used, user settings in ~/.m2/settings.xml are ignored (not tried, but I suspect global settings are too).
To me, this is wrong and poor. It would be much better if
# user and global settings were inherited and possibly overridden by the further settings file.
# multiple --settings <file> could be specified and a merge of settings could be composed by maven (eg, <profile> blocks would come from multiple files, <properties> would be overridden, considering the order in the command line).
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