CO has software that tries to detect multiple bookings.

The intent of a courtesy hold is so that you can firm up your plans before pulling the trigger. It is not to hold every combination of flihgts to see if something works out.

Not only is the online hold not supposed to be there (as has been mentioned), you're also using it against its intent.

Now had you held two totally different trips, on totally different days, you would've been ok.