As the two aircraft past the radar and are detected from different angles, the target on the radar display would get larger, and possibly even show as two targets for a short space of time. Unless of course that the missing plane manouvered itself around the 'shield' plane, knowing exactly where each radar is located. This theory could well be the truth, although it is unlikely. At the time though, I doubt the ATC's would take much notice of the size of a target on the radar display.Military radar are designed to pick up stealth aircraft, which have minimal surface area for returns to 'bounce' off of and show up on the radar. I'd like to think any military checking their radar data would look for anomalies in size, shape and speed of the aircraft it checked to be the missing plane. Or maybe the fact it showed up as a genuine aircraft, means these properties were merely overlooked.