Custom Blender Builds: How do they work together? If one developer of a feature (i.e Fracture Modifier) produces a full version of Blender incorporating their modifier, as opposed to just creating an add-on, and then another developer working on a different modifier (i.e. Manta Flow liquids) produces a full build incorporating their work, do I have to have two versions of Blender in order to use them both? I'm new to Blender and this whole Open Source thing, so I'm not sure how it all works.

Thanks,
Brian
In short: until both features are merged into the same build of Blender, you should treat those Blenders as two different applications.

I would suggest to use a file format like Alembic to bridge the different applications you want to use. For example, use the Fracture build of Blender to produce fractured objects, then export those to Alembic. You can then use the Manta Flow build of Blender to import those Alembic files and work with MantaFlow.