well, it seems like ya'll got a little off track... I hope not literally!
hmmm. yeah, no front plate on my car. I have 'antique' plates, I don't believe I received two.
Belts and breasts. I generally just use the lap belt when cruising around town. The harness is a hassle. If I do use the harness straps then tightening not too tight seems to be ok. Now on the track, different story. Full harness, with 'breast straps' pulled tight. Gives the 'flat look' a whole new meaning!!
If your girl is well endowed, then I could see how the straps pulled tight would be uncomfortable. For long trips, higher speed highway driving, I would use the full harness, just maybe not quite so tight.
I have been told (don't know if it's factual) that if I get pulled over, I could actually get a ticket for not being properly seatbelted, if I don't have the shoulder straps on, just the lap belt. Haven't been pulled over for anything yet.
If you only install the harness on the driver side, and the lap belt on the passenger side, it wouldn't be a problem.
Good luck!