I also replied to your other post. Posting the same question multiple times usually is not necessary.
You can use one ACS for both sites, with a couple of items to make note of:
- If you are using AD for authentication, be sure the two domains trust each other, and make sure a group mappinng is defined in ACS for each group of users you wish to enable.
- If the link between sites goes down, APs that cannot access the ACS cannot authenticate users.
I would recommend a second ACS server at the branch for redundancy, with each site pointing to their own ACS as primary, and the other as secondary. That way if the WAN goes down, or either ACS goes down, both sites can still function.
-Eric