Search the above website for "multiple resolution".A CrossRef member can go about multiple resolution in two ways.
1) They can deposit all resolution URLs themselves. They deposit multiple DOIs either on first deposit with everything else, or as an update to an existing DOI using a 'DOI resource deposit'.
2) They can 'unlock' their DOI so that other members can deposit additional resolution URLs for it. In this case, CrossRef can be asked to authorize only certain other members to deposit additional URLs.
For example, a publisher can unlock their DOIs and tell CrossRef to authorize a particular syndicating organisation to deposit additional URLs for their content. The syndicating organisation can then go ahead and deposit their own links for the publisher's DOIs.
There's some discussion of multiple resolution on the help site:
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