What I'm suggesting is that because they are both supercritical fluids they are going to permanently mix and this will alter drastically the phase shift of the co2
CP for air(nitrogen) 34Bar/-147degc
CP for CO2 72 Bar/26degc
So both are above the CP and therefore the standard phase shift for CO2 will not apply
All supercritical fluids are completely miscible with each other so for a mixture a single phase can be guaranteed if the critical point of the mixture is exceeded. The critical point of a binary mixture can be estimated as the arithmetic mean of the critical temperatures and pressures of the two components,
Tc(mix) = (mole fraction A) x TcA + (mole fraction B) x TcB.