How to introduce new dog?

i have a 5 year old male presa canario and it has been our only dog since we gave away his sister at 3 months. we are bringing in my cousins Argentinian mastiff full grown 3 year old male. the mastiff had been involved in a defensive scruff " 1 time( it wasn't serious just a quick growl and snap), no dogs were hurt but my presa is 135 pounds of muscular dog and he can hold his own. they have briefly met once before and it went OK apart from high alertness and a few barks. they WILL live together whether they like it or not but i just keep them from fighting for the stay, about 1 month. my cousin left for vacation for mexico and there is no other place for the dog to stay, we WONT take him to a kennel and most of our family lives nowhere near us. they are both big, dominant dogs but idk if i should just leave them be because i think that since dogs are pack animals they should just establish whose boss. if you are not going to answer and tell me im a horrible person of say something about my cousin then go finger yourself, just because you like lapdogs and think bully breeds are bad doesnt mean everybody does.