Merging 2 distinct exchange environments - HELP!!!!!!!

recently, ABC company bought DEF company. DEF has an exchange environment and ABC company has an exchange environment. there is also ABC.com and DEF.com ... ABC and DEF are on opposite sides of the 2 different countries. My problem ...

I am responsible for DEF companies exchange enviroment, and DEF.com .. i need to know how to get the 2 environments to speak to each other, and point to each other to merge the mail and internet environments and ANY help would be fantastic!!!!

Thank you in advance