Exchange in other domain

Has anyone implemented Outlook/Exchange in a separate AD domain? We have two companies that merged. Each company has a Windows AD domain and Exchange email. Management wants a single Exchange email domain but two separate Windows AD domains. Is this feasible? And if one Windows AD domain houses Exchange then what happens to the other domain? Can you put the Exchange server/smtp information into the domain that doesn’t have Exchange installed? For example: you create a user on the domain that doesn’t have Exchange installed anymore, how do you create the user and have the email acccount and information filled in automatically?