I think as long as you devote time and training to the first dog and wait until it has sufficient training and socialization before adding the second dog, it should work out okay. Make sure you sign up for obedience classes!
I would NOT suggest getting two puppies! Either get one puppy and raise and train that one well before adding a second or start with an adult, reasonably well trained dog and add a puppy or adult dog. Male and female pairs tend to work best, but individual dogs have their preferences.
But... weren't you fairly pressed for time in terms of exercising a dog and training a dog?
Two dogs is not easier than one, they don't exercise and train each other. You will need to exercise them together and separately, train them together and separately etc. And you have to consider, esp. with those breeds, that there may come a time when one or the other matures and decides he/she doesn't like the other. Then you are stuck with crate and rotate which takes even MORE time OR rehoming one of the dogs that you've likely become very attached to.
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.