This is quite an old question now but I'll have a go for future reference. It depends on which DOORS NG version but assume it's at least 6.0.3 because before this version your options were not worth talking about.

• Both project's streams must be in the same Global Configuration stream.
• Both project's streams must have the same link types & link constraints (this can include user-defined link types).
• The link creator must be a member of both projects and of the Global Configuration.
• Both projects must be opened in the Global Configuration stream context.
• The link creator needs to have a role/permissions in the projects to be able to create links.