It would be possible to design a regulator that feeds two 5 V loads from one 12 V supply, and the current taken is the maximum of the two loads.
However, you certainly can't do it with 7805 regulators. You need to allow for the two loads taking unequal currents, even if they are rated the same. Two phones will become fully charged, and so take less current, at different times.
If you had two devices on a regulator like that, the earths of the two devices would be at different voltages, so if you connected the audio output of one to the line-in of the car radio, you could get problems.
It is far easier to find a good switch-mode regulator that will reduce 12 V to 5 V more efficiently, and to run the devices in parallel from that.