If it's the same as the Xbox version, it should go this way:
Select the defensive player that you want to double the receiver.
Press "Y" for Coverage Audible, which brings up the cursors over the opposing receivers.
Press the corresponding button to the receiver you want doubled and then tap right on the D-pad.
That causes the corner (or whomever was assigned in the first place) to double the receiver with the guy you selected defensively. At least that's how it was on the Xbox.
[quote name='my boy blue']Check on the defensive tutorial if there is one. I know they explained it on the regular Xbox version.[/QUOTE]
alas... the biggest problem of madden 360 .... it is pretty bare bones.. and there pretty much aren't any extra features such as mini games and tutorials. The instruction booklet itself is like 3 pages long.
Also.. the controls are switched up in the 360 version... and actually are more like nfl 2k5.