Dual Comparators connection that affect the design
The LM2903 is a bipolar comparator and you can safely leave all the unused connections open. 

If it's a CMOS-input comparator then it would be best to tie the inputs to some known voltage within the power supply range. 

If you are doing something especially critical (sub-mV) you may wish to bias the inputs so the output is in a known state (which means the inputs are at different voltages within the common-mode range)- because the bias networks tend to share a few parts there might be a slight effect of one comparator on the others. Unused output(s) can always be left open, whether open drain/collector or push-pull. You can ground unused open drain/collector outputs if you want, but it doesn't matter.