Do I have to use both the R and L main output on the Behringer Xenyx 1002 mixer to the channel 1 and channel 2 input on the Crown CTs2000 amplifier? I would rather use just one output on the mixer and one input on the amplifier if possible. The amplifier has a "Y" switch to put the same signal on both channels. I am hooking up a 70v system so I just need a mono signal. I guess I'm asking if that would hurt the mixer in any way?
No, you can send the left channel from the mixer to the amp and link the two amp channels with no problem. The result will be a mono signal in two channels as you describe yourself.