Recent post on 2 puppies and housetraining -- gone missing
Four months old is very young still, and you are seeing typical issues with housetraining and definitely should not discontinue rewards. :) If anything, perhaps the reward is too 'low value' -- the better the treat (eg say the commercial dried liver treats are very desirable to a puppy, as is a bit of hot dog etc) the housetraining lesson registers far more than with say a piece of dried kibble. Housetraining with two generally takes considerably longer, as well.
I agree with Nicki on the extra challenges of two puppies -- trainers advise not to get two puppies together, especially two siblings. Dog trainer, Kelly Gorman Dunbar, noted yesterday in a discussion on this, that it's like having THREE puppies -- because you really need to be raising the two mostly separately -- so they need to each get concentrated daily time with you separately, and then, supervised time together, hence; three puppies.
I'd actually totally separate the puppies as much as possible every day, for housetraining and at night. Give them some play time together but other than that -- they should be separated.
You can and should read up on how to raise two puppies to avoid problems. It isn't that this is impossible -- it's just that you have an enormous amount of extra work to do. It really annoys me that breeders ever sell siblings to people without explaining all of this in detail. :(
On the plus side: like many people, I've just raised two sibs of another breed myself. so it definitely isn't impossible :) -- but it WAS a major challenge as each needed hours daily of focused time away from their sibling. They also needed training classes separately. It was very time-consuming for my partner and me. Housetraining definitely took longer than previous single puppies we have had.
Keep in mind too, that you have dogs of opposite sex and the female can go into heat as early as 4 months. This is going to be another challenge and a responsibility, as you do not want to risk an accidental mating especially between siblings, in this already genetically-challenged breed. It will require a fair bit of managing (and again, separation) as they get sexually mature -- this is generally around 6-9 months but can be earlier -- and best to talk this through with your vet on what to look for, when to split them (once a female is in heat she needs to be separated by two doors at all times from the male, for 4 weeks, absolutely never risking them near each other). Your vet can advise on managing and/or neutering at a safe time.
I've added some links on the two-puppy issue below :D If this seems overwhelming -- and for some people, it will be a slowly dawning shock! -- you may wish to weigh up all the pros and cons of raising two together. If you want to see what it is like to focus on just one, perhaps the breeder would take one back and let you try a singleton as an alternative just for a week as a trial. Some people find they prefer to stick with one, once they consider the realities that perhaps weren't really presented to them before they opted for two. I know those are hard options, but the end goal of course is for owners to enjoy the experience of having a dog, and not feel overwhelmed; likewise for every puppy to get the attention, time and training it needs. You may feel alternatively that you are well able for this challenge, having read through some of the advice in the links; we'll certainly be happy to offer advice too here!