My partner is picking up his bullmastiff pup (dog) next week. He will be 8 weeks old. We have a 7yr golden retriever (bitch) which we know the 2 of them will be ok. Now, i want to get a golden retriever pup (dog) around august time, so the 2 pups would grow up together. Could anyone out of experience  tell if we would encounter any problems with "2" dogs living together....e.g...dominance issues etc. We know retrievers and bullmastiffs are great dogs with children and other animals, but bringing the 2 dogs of the breeds together in the same house , are we asking for trouble???? Many thanks for your replies in advance.......

I don't think that you should have any problems, however it all depends on how you raise them.
My first dog was a bully, he was 2.5 years when i had my second dog a bitch.  We had 3 other girls no problems - then i imported a male, who when released from quarantine at 10 months was bigger than the bully.  We didn't have any problems at all.
But i was always top dog so neither of them felt the need to prove anything.
Treat them both the same and fairly and always remain the boss, and they will be fine.
Sharon