>
> Then typing TAB could insert the arrow separator when it doesn't exist yet.
> When it exists, then TAB could jump to the second field. And fields could
> be highlighted using the same faces as customization widgets use for fields.

If you have two input fields which you can edit before submitting then the whole arrow
thing is obsolete, because you don't have a single field where you have to insert
a marker to separate the from/to inputs.

You have two fields which can be edited separately, they expand as necessary to show
their contents in full, you press TAB to switch between them as needed and
you press Enter to submit when ready.

See the attached picture for two mockups: first the initial state and then a state
before submitting when the user filled the inputs.