Others know better than me the actual logistics, but it is no problem at all. I've seen a thread with someone booked as (eg) Mrs. Smith, with a valuable instrument booked alongside as Cello Smith.

I've actually been on a flight shared by an orchestra with two cellos occupying seats (assume they were paid for as the flight was full). My seat opponent and hi did chuckle at the thought that they both could have easily contained bodies - our morbid soh I suppose !

Rich