That dual bridge use is for two bridges stacked, not across a table for support. So you can put a bridge on the table, then put the other bridge either over it or in the grove, and then the shaft will be on the top bridge.
This is from WPA " The purpose of the mechanical bridge is to replace the hand bridge for shots that are difficult to reach".
Going by that description, you can't use the bridge as a support to lay across the table since you can't do that with your hand bridge.
If you can only use the bridge to do what the hand can do, you wouldn't need a bridge.
