Border collie and BC mixes are not your average dog in my opinion. They are extremely, extremely intelligent dogs. Bred to work and be independent thinkers, they need activity, and lots of it, and do best when they have a job or sport they are active in daily. Left to their own devices they get bored very quickly and so yes, they will find ways to entertain themselves.
I have a BC mix and she is far and away the smartest little dog I've ever had, that little brain is fast! She's a ranch dog so she's on the go most of the day but at the end of the day she's still ready for a round of ball. She just never quits!
And yes, training two pup's at the same time is a big challenge. You need to work with each one separately each day, away from the other one. Lots of one on one time. When they have downtime outside put them in a secure area just for them with lot's of puppy safe things for them to play with. Teach them tricks, teach them to play ball, great way to burn off energy. Use lots of positive reinforcement. I have found these dogs to be very easy to train, they want to know what you want from them and they want to do it. The only correction I've had to use on my young one is a quick tug on the leash and/or a loud "psst" to get and redirect her attention. Older dogs may sometimes need a firmer correction but with pup's that's usually not the case.
Good luck with yours, they are very cute! If you are willing to really invest plenty of time in them in the next couple years you can turn out a couple of really nice dogs that you will really enjoy.