How easy is it to book ADRs for 4 and then change to 2 later if needed

It's easy to change a reservation to make the party smaller. Going the other way is less easy; we did it once to add one person and ended up with a later ADR.

I think you should be able to make two different reservations at the same time, but don't forget to cancel the one you won't use more than a day ahead. And doing it as soon as you know whether you're a group of 2 or 4 would be best.

I agree with CCP, going down in size is always easier than going up in size.

As for the double booking, as long as the reservations are in different names it should be fine.

I agree with the previous ladies, you shouldn't have an issue switching from 4 to 2, just call and it shouldn't be a problem. However, when I've tried to book two reservations on the website for even around the same time it has sometimes worked and sometimes hasn't. If you have issues I would call and try and make the reservations over the phone and they should be able to help you out.