The best ones I have seen are an L and usually sit on a corner lot, with one side facing one street and the other facing the other street. You can do an arrangement with the two garages connected and forming the L. The fact that the two units aren't identical doesn't look at all odd.
Just a piece of advice from someone who has adult relatives living with them--and your DD is a good deal more independent than mine. Also, my MIL died, leaving me with an apartment in the basement of my house and which I would not want to rent to anyone outside of the family. Your daughter might stay for 6 months and decide to move, and what would you do then? With totally separate apartments, such as you are considering, you could rent it. But do you really want to live in a duplex? (I wouldn't mind it, but DH would not want strangers living in such close proximity). So now we come to the advice: don't do anything that would leave you in a position of living somewhere that you might not want to be if it wasn't your daughter living next door. The fact is, she will probably meet someone and get married. If not sooner, then later. If you're willing to rent out the other side, this could be a great arrangement.