I assume your install includes a high-to-low-level converter to connect your amp to the factory head unit?
It is possible to have 2 head units in your car, but it seems a little pointless. Either you're happy with the factory unit or you're not...if you want more functionality, you might as well replace it rather than trying to double up. You will lose the read-out in the dash and the auto volume adjustment, and you will need an adapter to keep the steering wheel controls, but you will get all the other stuff you want.
It's fairly common practice to double up head units in BA/BF Falcons (their head units are integrated into the dash so removal is difficult) so a little searching along that line might give you more info if that's what you want to do. Perhaps you can feed the factory head unit's outputs into the new head unit's Aux-in port - but I can't see why you'd bother.
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