What gorgeous fur babies!
We don't have a bull mastiff--but we have a large rescue dog that had the same problems around 8 months. I couldn't bare to leave him in a crate all day while we were at work so I understand where you're coming from there. As some suggest we gated him into "pee friendly" portions of the house even before the 8 month mark. He didn't have an accident in the house anywhere after 3 months except ONE particular spot. He peed there every day for a week even though I cleaned it up! So frustrating. Someone told me to clean it with a special cleaner so he couldn't smell it--after that he had no issues with going in the house. It was frustrating when he did go though because we had someone come and let him out every day and bring their dog for him to play with. So it definitely wasn't that he was inside too long. I also usually came home once a day so he was never alone for longer then two hours.
The BIG problem however was the chewing---anything we could think of he destroyed--the table cloth, a cake on top of the fridge (still trying to figure that one out!), shoes that were put up so high I could barely reach them--nothing was safe from such a large dog.
We eventually took him to puppy school at 9.5 months. The teacher suggested getting him a back pack. WHAT A DIFFERENCE it made! We had always taken him for bike rides, runs, or walks at night, and he had puppy play group once a week but such high energy we couldn't drain it. After I had him chase me on my bike for 5K with some weights in his back pack he was exhausted! I would do this in the morning usually so he just slept until I came back.
I can't speak to it being an issue with both dogs in the same place however, we just got another dog briefly (while someone had a baby and needed to move) for a month and the two dogs didn't have any problems in the house during the day--except most nights they were so tired from playing with each other they didn't want to go for the bike ride!
I'm no expert but hopefully I've provided some suggestions