One of the main reasons that trainers do not recommend getting two puppies at the same time is that housebreaking can be very difficult. Puppies pick up bad behavior from one another just like children. When one regresses, the other one regresses, too.
Are you training them separately? That is the key to success with two puppies. They need separate crates, separate training, separate socialization, etc. You have to go back to Potty training 101 with both of them. Take them to their pads separately at regular times, after meals, after a nap, etc. Praise and treat if they potty. If not, back in the crate and try again in about a half an hour.
At seven months, Calvin is officially a puppy teenager. Potty training regression, not listening, challenging your authority are part of this stage. You will have to work extra hard with him during this stage. Bella is just a few months behind him so you will face teenage challenges with her, too. Work really hard with her or Calvin will pick up her bad habits and you'll be back to square one again.
Great tips in this article:
Two Puppies or One? - VeterinaryPartner.com - a VIN company!