With a 600EX-RT, the simplest starting point is usually to use Canon’s built-in RT radio system rather than adding separate triggers. You can either use a second 600EX-RT on-camera as both master and bounce flash, or use an ST-E3-RT transmitter if you only want remote control.

A very practical reception setup is:

• one flash on-camera for bounce/fill
• one off-camera flash near the edge of the dance floor as a remote light

This gives you flexibility and lets you adjust the relative power of the on-camera and remote flash from the camera.

For first dances, two off-camera speedlights can work well. A common starting point is placing them at roughly 45 degrees to the subjects from opposite sides of the dance floor. That gives more dimensional light, but it can be less flexible if you move around a lot.

For general guest coverage, large modifiers or stands can get in the way, so keep setups simple and unobtrusive. Start by practicing with one remote flash, then add a second once you’re comfortable with placement and power control.