None of the Canon flashes can be used simultaneously as slave and master. The 550EX can only be used as a master on-camera (or connected by a TTL cable to a hotshoe), or a slave off-camera. It cannot be an off-camera slave and an off-camera master at the same time. The same is also true of the 580EX, 580EXII, and 600EX-RT. In addition, once the PC port on a Canon flash is used, its optical master capability is also turned off. The flash will only "listen and talk" on one thing at a time: the foot, the smart optical sensor, or the PC port. You need to get an additional RF-603II to be a receiver for your 430EX, and consider the on-camera RF-603II as your master transmitter.
If you'd prefer to have more than manual-only triggers, you could use a 90EX or ST-E2 as your on-camera master (assuming you aren't shooting with a 600D, 60D, or 7D or later body which has one in the pop-up flash), and use that to control both of your EX flashes via Canon's optical wireless scheme. Or you could switch to TTL-capable triggers, like the Yongnuo YN-622/YN-622-TX or Phottix Odin triggers, but again, you'd need at least three units: two to act as receivers, and one to be the on-camera master transmitter.