AD 2003 Native
AD 2008 Mixed
CompanyA acquires CompanyB.
Single forest with two child domains (CompanyA.parentco.corp and CompanyB.parentco.corp).
Single Exchange organization to host email boxes for CompanyA and CompanyB users. The users would need to default to their respective external domain addresses (companyA.com for CompanyA.partentco.corp and companyB.com for CompanyB.parentco.corp) but have the ability to have the other external domain addresses added as needed on a case by case basis.
AD is to be 2008 and Exchange is to be 2010.
CompanyA desires for CompanyA.parentco.corp to host its users and computers and CompanyB.parentco.corp to host its users and computer.
What is the easiest way to make this happen? What is the safest way to make this happen? New hardware is a given and will be purchased, but not until a plan is in place and approved.