A service that is repeated by another physician is normally billed with modifier 77, however in this case this may not be appropriate because CMS publishes an MUE value of 1 for this code, which means only one service per day is normally performed.
Did this patient actually receive two separate medically necessary hyperbaric treatments on the same day? And this was done in the office? That would be very unusual, but if in fact it was the case, you can bill both and then appeal the denial with notes. But if there was only one treatment, then your physicians should not be billing two separate charges for this.