With "2" the only way to get money is to either close part of the software,
Or, as in our case (x264), you keep everything open, but open source version is GPL, thus limiting its possible use in proprietary software packages, whereas the commercial version is not (despite them being line-for-line identical).
This only works if the project's license is copyleft, of course.
That's what contributory license agreements are for: any project that plans to follow such a route needs to get all their developers to sign one in order to get the rights.
In my experience this was some effort, but not overwhelmingly so, IMO. This would probably be more difficult for a larger project, but other projects have done similar things (e.g. VLC's planned relicensing of libVLC to LGPL).
Or, as in our case (x264), you keep everything open, but open source version is GPL, thus limiting its possible use in proprietary software packages, whereas the commercial version is not (despite them being line-for-line identical).
This only works if the project's license is copyleft, of course.