I'd avoid doing this. Not for performance reasons, but because it makes for a poor database design. Each table/database entity should be created for one purpose. To blend together two tables blurs that purpose. I suspect that you might also have problems defining two foreign key relationships on the child column.
I don't think you'd gain much performance. If anything you'd probably lose performance, as the tables would be longer, resulting is more rows to scan for joins, where conditions and the like.