Nice idea, however I'd hope that modern radar is able to detect two aircraft of that size flying close together. Unless the perpetrators know the various limitations of each radar installation?
It's a matter of radar resolution. Radars can't distinguish flies apart from each other, so obviously there are technology limitations anyway.
However thats not the case nowadays. Even if you had a nasty radar from the 80's you see that the trace is twice the size it should be. (We'll I'd hope)
I'd assume that military radar ops would be looking for this, as this is a great way to sneak assets about.