Actually, Workflowy’s implementation of board view is completely different from the “Split screen” request on Dynalist’s Trello. Basically, each top-level parent item in an outline is turned into a column, and its immediate children become the column’s “cards”, with their descendents inside the cards. It could be interesting for people who like the kanban interface.
It’s not the same as the idea of having multiple panes/tabs/windows that can each have any document or section open and allow you to drag things between them, which is the top request on the Trello board. Apparently, Workflowy is also planning such a “panes” feature, but doesn’t offer it yet.