i think it would depend on how often you and your husband drive your baby around. if she rides with you most of the time, and your husband only occasionally, then i'd only get one. if you guys switch off a lot, it might be worth getting two.
my only concern here is that if you only have one seat and ya'll switch a lot, if you are running late and are rushing with her out to the car, and oh look there's the car seat sitting by your car because your husband had it last but didn't have time to strap it into your car (my husband does that to me a lot!), and because you are running late, and you're annoyed with hubby because this is the fifth time this week he has done this to you, you don't spend that extra couple of seconds making sure the seat is put in right, or tightening the seat down all the way, or making sure the seat belt buckle clicked all the way, or whatever..... it just makes it that much more likely that she could get hurt if you are in a car accident... does that make sense?
SOOO, if she primarily rides in your car, and only occasionally with your husband, i'd definitely stick with one. if ya'll divide the time pretty equally, then it might be worth getting two. just FYI, we got the COSCO brand $50 one from target, and they've always worked great!