It will happen that people submit several patches that are interdependant.
Linux Kernel workflow is, as reflected by git-format-patch, to send several requests tagged [PATCH 1/5] [PATCH 2/5] etc.
Our way of reflecting such dependencies in FreedroidRPG (which uses ReviewBoard) is to manually tag the patch titles.
It would however be possible to do something more clever, such as a tree view, but the complexity of that means this is probably out of the scope of ReviewBoard as a lightweight tool.
However, it would be possible to add a "depends on" field to each ticket, just like the "bug" field. It would be a good start and help us significantly.