Should be fine and makes keeping them in sync easier. Before making a commit or merging a branch one can run the complete test suite including integration tests to validate a minimum consistency.

If fact I'd do this by default, unless they were huge or some other reason like different teams or companies were developing them. Or perhaps one part is mature, and the other is rapidly developing. But, these should be uncommon cases.

(I'd still keep associated projects that aren't directly coupled separate however.)

Should be fine and makes keeping them in sync easier. Before making a commit or merging a branch one can run the complete test suite including integration tests to validate minimum compatibility requirements.

If fact I'd do this by default, unless they were huge or some other reason like different teams or companies were developing them. Or perhaps one part is mature, and the other is rapidly developing. But, these should be uncommon cases.

(I'd still keep associated projects that aren't directly coupled separate however.)