Once you have detonated the first charge, move through the sewer tunnel to the next area, where you will find the next anti-aircraft machine, and three guards. You will notice that one guard moves close to you and sits down. Both spies should quickly enter the ground level, with one of them hiding to the right. This will draw the attention to the first spy as he kills the seated guard.
Kill the Three Guards and Detonate the Second Anti-Aircraft
As soon as the first guard is killed, things will get a bit messy, but as the attention is focused on the one spy, the other can easily take out the other guards from behind. Keep in mind, there is a time limit to detonating this next charge, so move quickly in this area. Once the three guards are eliminated, there are no more threats in this area, so place the charge on the anti-aircraft as you did on the previous one, move away, and detonate it.
Once detonated, you'll hear a humerous exchange between Sam Fisher and one of the terrorists, keep moving. You'll find a small rectangular entry in the ground to a sub-level sewer system. Follow that system, and when you arrive at the top, head to the left and go into the room on the right. Quickly turn the light off (the switch is on the right as soon as you enter), restore your health via the med kit, and save the game.