Looks like the renderer (or actually its ClickableRenderer superclass) only supports single-click events.
I guess you could create a feature ticket for that and hope for the best. But, that could take some time to get done, so if you really need the feature, I’d recommend implementing a custom renderer based on either ImageRenderer or ClickableRenderer. It’s maybe some effort, but hopefully not too much. You maybe need to duplicate RendererClickEvent as RendererDoubleClickEvent, or maybe you could also generalize it to handle also double clicks. You can probably get the double-click events from GWT. Then add the RendererDoubleClickHandler to the handler manager.