I'd like to book my parents first class award tickets. The problem is of course that only one seat is usually available.

I've already booked them in business class as they would like to travel together and in case we can't get two first class tickets.

I was thinking of booking mom in the first class seat that's available now and then check closer to departure if another one opens up.

Is it okay for her to have two tickets on the same plane or will the system auto-cancel one of them?

Thanks for your help!