... will corona be twice as fast in rendering?
TL:DR
It depends.
In very simple scene scenario you actually could get 200% speed. But it may heavily vary depend on scene complexity.
Assuming extra machine will be exactly in same configuration as present machine, usually you will be somewhere close to 200% speed gain,.
Just to give you example I made quick comparison for you:
Every set being rendered 3 times, and best times were taken. All render nodes have exactly same configuration. I made only 4 renders, since I do not have a lot time on my hand, but it should enough to get a picture.
Timing: Please notice different amount of passes every picture have. I could be wrong (ask Keymaster for technical specifics) but I believe it does happen due the fact when render is finished, all nodes start dumping their work data from last update and render gaining few more passes. As you notices time is shortening for every added node, but gain is start dropping rapidly after 3rd machine were added. It took only 1 extra node to get double speed, 4 to get triple and I reckon to get to quadruple speed you will need 8 machines etc. 16, 32 ... But again do not use those numbers as guidance, since scene I used is extremely simple, and you probably going to get different figures on different scene.
Most benefit from network render gained during long lasting renders or multiple frames. When making short test renders, sometimes extra nodes are not fast enough to participate in render process due various factors. Times could be improved by: adjusting sync interval, changing amount of data transfered, improving physical connection between machines, etc.
Cheers!