You said in another thread that you can only provide 30 minutes of exercise per day, which isn't really enough for one of these dogs, let alone two. Raising two puppies together is also difficult to do well, as they can bond to each other and not to you, and you really need to bond with a dog to train it properly. You want a "guard dog," yes? What you don't want are two large, under-stimulated, potentially aggressive dogs who'd rather feed off each other's energy than listen to you.
I also must point out that you're balking at spending the $1200+ for a well-bred cane corso. If you can't afford that, you definitely can't afford a bullmastiff as well. And these are both breeds that you need to get from a really good breeder to assure their health and temperament, especially if you're going to have them around a 10-year-old every weekend.
If you got one dog, and then in a year or two added another (as long as you have the time and money), that would be okay. Opposite genders seem to work best, and I don't think it matters which gender you get first.