The strobe line transfers the data you have clocked into the shift register onto the output latches, it's the output latches that connect to the outside world. So strobe should be pulsed once at the end of sending the clock pulses.
You send 8 clock pulses for each daisy chained 4749, so that's 16 if you have two.
The first bit you clock in ends up at the very end of the shift register.
The output enable activates the output. The outputs will be on all the time if this is high. If it is fed by a pulse width modulated signal (PWM) then this turns all the outputs on and off rapidly. The ratio of on to off time then will control the brightness of any LED attached to the output.
I hope that clears things up.