Modern collaborative visual tools like Miro sidestep the concurrent development problem by being live multiplayer and by essentially having no version management at all.
Most visual BPM tools that I'm aware of try to mix flowchart style programming with e.g. Javascript on activity nodes, but they fail provide any developer-time syntax checking or completion. They also tend to serialise to XML which is unworkable in practice for diff/merge of the visual logic.
In a previous job we developed an in-house flowchart based language that embedded Javascript. We put the effort into writing a first-class Eclipse plugin that had syntax checking and completion across all graphical and Javascript constructs. It even had interactive debugging that interleaved graphical and textual single-stepping. We never solved the diff/merge problem in a satisfactory way.
