According to the FAA Flight Information Publication "Alaska Supplemental" booklet (FAA Product ID: ACS), in the Notices portion of the booklet, under the heading Communications and Position Reporting, Section III, it states,

"Normally, each oceanic radio station continuously monitors all assigned frequencies. If en route HF communications fail, every effort should be made by the flight crew to relay progress reports through other aircraft. The VHF frequency 123.45 MHz is for exclusive use as an air-to-air frequency communications channel (see paragraph IV.B. below). In emergencies, however, initial contact for such relays may be established on 121.5 MHz (the emergency frequency guarded by all aircraft operating in the oceanic airspace) and transferred as necessary to 123.45, etc...."

123.45 MHz is indeed a legal air-to-air frequency for oceanic flights.