6 months can be a trying age for puppies, as they often start pressing their boundaries a bit at that age, mine certainly did! And working with litter mates probably makes that quite a bit more challenging as well. But it's also a crucial time to stay the course on obedience. Be firm and don't let them "get away" with non compliance!
If it were me I'd be working them both independently and together. When you have 2 dogs it's important from my perspective to teach them to discern when you're talking to each of them. I did this with my 2 (1 GSD and 1 Chihuahua) by having them do commands together...not sure in hindsight that that was the best approach though. But it worked out. After they were both able to follow simple commands (sit, down, wait, for example) I started giving them individual commands, like having one down and stay and then recalling the other. If they both did their task correctly, both got a treat. If not, the one who did got a treat, then we'd try again.
I would also recommend teaching them a more formal heel. For that do it inside with no distractions initially, and use food as a lure to get them in the proper position, then treat and praise. Practice that a bit, then take a step (sideways, forward, or even backward) and give the command again...lure them if needed with food. Make a game out of it, mix up the direction you step to keep them on their toes, and most of all have fun with it. Reward and praise profusely when they get it right! Make sure they "get" that heel means a certain position in relation to you before you start walking anywhere. Once the get that clearly, you can start walking a few steps...then more and more.
When you stop they should sit. But dogs tend to want to sit facing you, so it's likely they will be by your side, but with them facing toward you, which is not the proper heeling position. To address this I used the walls in the house. By stopping always with only 6 or 8 inches between me and the wall, the dog had no choice but to sit properly aligned. With my dog I practiced this a lot before venturing outside for practice.
Anyway, getting your puppies to mind you away from home, and particularly in places like a dog park with tons of distraction is a process. It won't happen overnight. And to achieve that level of obedience YOU have to be very disciplined. Don't give a command you can't enforce. Don't give a command and EVER allow non compliance. No need to be angry or mean about it, but firm, fair, and consistent is the mantra (to which I always like to add insistent!).
And throw in some fun, keep training sessions short, focus on a single behavior, and follow up your training with play time! Good luck, and again, gorgeous dogs you have there!
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain