Didn't they talk last season about giving an extra test in case of driver replacement from a safety perspective? Or did that not pan out?
Either way, I guess they are telling it like it is - Senna gets some miles in case Heidfeld doesn't work out or is also injured, and Heidfeld gets some seat time to learn the car. If these are the only two drivers they are trying out, I would say it is certain that Nick get's the drive.
Well it makes sense not to be hasty, and pick the fastest. It'll probably be Heidfeld.