My questions:
- If I have to do the spending, and then wait 8-10 weeks for points to reach my account, am I risking not having any seats available on which I can use miles? I don’t know the time-frame on which these seats get taken up.
I would definitely not expect to have those RDU-CDG tickets available at 20k AA miles by the time your bonus clears, especially at that spending level. (I frequently make big spending bonuses as I have a lot of reimbursable business expenses...)
- Does either AA or United offer additional flexibility that I should be aware of should I need to move my flights up or back a day (which there is a not insignificant chance I will need).
Generally, both airlines are actually more flexible when it comes to mileage tickets -- provided that seats are available at the price you paid.
- Can we both sign up for a card (assuming we can reach the spend limit on each)?
Yes, and you should. You should start reading the Frugal Travel Guy blog -- they talk often about situations just like yours.
You can start reading some others, like the Points Guy or Frequent Miler, as well the Flyertalk forums if you want to go deep. By then, you're getting into hobby territory (where I am at).
- How can I make this flight to St. Louis also fit into this plan?
If your plans are mostly to fly domestic and you're based out of RDU, look into getting the Southwest cards as well. Much like starting investing, you should think about your goals FIRST -- i.e. domestic travel, international first class luxury travel, etc -- and make decisions based on those goals.
The best course of action, in my opinion, is to start accumulating points/miles NOW so that when an opportunity comes up you'll be able to pounce on it. I don't hoard points, but I accumulate as many as possible all the time so that I can, say, fly to Montreal next weekend with 2 friends for $10 a piece. Or make last-minute trips to visit my mom back in Texas, etc.