Only if both drivers were in the exact same place in the car literally, even 5” centre to centre will change the reflections which will then potentially influence phaseOne question (well, actually it's more than one...). Isn't this phase shift of the cars acoustics done to all drivers at the same time? F.e. in the area where you pass over the duty from midrange to tweeter?
If yes, the relative phase would stay the same even due to the induced phase shift by the car, right?
In this case, an allpass filter as created by either PhaC or the Helix phase control would still work as intended.
this is why driver positioning and crossover freq have to be chosen very carefully
Playing a midrange below beaming for example doesn’t mean you can mount it anywhere as some people would have you believe, an inch difference… (aiming will change off axis response and also move this centre to surface distance in smaller degrees so does need consideration also) up/down/back/forwards can make a decent sized difference to the response in both phase and also magnitude response 👍🏼