The ACS server can be configured to work with two windows domains? to authenticate users that belong to the domain called "a" and "b", the protocol to authentificate it is 802.1X in a Wireless Enviroments (WLC4400+ACS4.2+Two Windows Domain).
Under certain conditions, yes. You have to have trust between the domains, and depending on whether you are running the ACS on an appliance or a server, there's certain configurations you have to do to make it work with multi-domain authentication.