Master Slave

Can I use a 550EX as a master and a 420ex as a slave but, have the light from the 550EX less intense than that from the 420EX. In other words, I would like the 550EX which will be on the camera to be the fill and the 420EX which will be off the camera as the modeling light.

Yes, you can. You'll need to put the 420EX in Group B. The 550EX will automatically be in Group A. Then set your A:B ratio to whatever ratio you want. For example, an A:B ratio of 1:4 will give you a light output from the 550EX which is four times less bright (or 2 stop less) than that of the 420EX. Or, conversely, the light output from the 420EX will be four times brighter than that of the 550EX.