Hi Vivien
My husband and I bought two puppies (litter mates) at the same time for the same reasons you have bought two puppies. Yes, it certainly is a lot of work and an extra challenge with litter mates.
We found it a challenge to train both boys at the same time, however, it is not impossible. The method we used to teach the basic commands such as 'sit', 'stay', 'drop' etc was to separate them at dinner time. My husband would take one dog outside on the verandah and I would stay inside with the other. As both were hungry for their dinner that is what they were focused on rather than being separated. If your dogs are food motivated (as ours are) and are separted for dinner time, you might start to have some success. We also kept their training sessions very short at this stage, no more than a few minutes.
I will add that we were also taking the boys to puppy class during the first few weeks they were with us (they are now 6 months old), so we were shown the method of teaching the basic commands using food. I can tell you that both boys will do what they are asked now when they are together. I wish you the best of luck.
fluffyblackpoodle