The second defender must do two
things at once:
Mark the second most dangerous attacker (the one
closest to the man with the ball) (medium-to-close marking)
Protect against the first defender getting beat
So being a good second
defender is all about positioning, just as being a good first
defender means being able to stop an attacker one-on-one.
Stay close enough to your own man that you get there
very soon after the ball if a pass is made.
Watch for the second attacker making a run into space
Get the angle right to support the first defender
Ideally, be close enough to the first defender that an
attacker beating the first defender runs immediately into you before he gets
the ball back under control
Be prepared to become 1st or 3rd
defender if the attack switches focus
The Second Defender
A. Well positioned defender.
D2 is the 2nd defender. He has taken up a
perfect 2nd defender position:
Backing up D1, and can quickly close if A1
dribbles past D1
Close enough to close down A2 quickly if A2 gets
Blocking the passing lane from A1 to A3
B. Poorly positioned defender
2nd defender is playing almost level with D1.
This brings him closer to A2. But A1 now has three new options. He can:
Dribble around A1 to the outside, where there is now no cover
Make a through ball behind D2 
Pass the ball in to the target man up A2 front
The 2nd defender, while only a few yards away from
being perfectly positioned, is now almost useless.
So as second defender, you need to make small adjustments to get the
positioning right. This critically important. Typically, youth players do
not adjust until after D1 has been beaten. That is too late.