Can we install 2 Domino server in a single internet domain? Can we install a Domino and a MS Exchange server in same internet domain?
Yes. Just specify IPs of your servers in DNS (A record) for (for example) domino1.example.com, domino2.example.com and exchange.example.com.
I wouldn´t install Domino on top of MS Exchange. MS Exchange is a ressource intensive application. Our Exchange system is beside the Domino environment, the reason is that MS Exchange takes ALL the RAM he could get which caused issues on the Domino system. Its also hard to reconfigure the system because the SMTP port must then be changed to different ones which made the routing from external more complex.
But could it be you are trying to migrate from Domino to Exchange or from Exchange to Domino? Then the best way would be to implement something called a "Smarthost". So if you then send an external email to Domino, domino will check if the user is in the adressbook, if not, it will send the email to the smarthost e.g. your exchange server. With that construct you can "install a Domino and a MS Exchange server in same internet domain".