Don't just assume that they're both eating. One certain way to tell is not to leave food available at all times (though water should always be down, of course!).
Have a feeding schedule and stick to it, the same time each day. Give each cat his own plate. Sit down quietly and watch them eat. There'll be no more guessing as to who's eating and how much.
Often one of the first clues that there's a health problem is when a kitty isn't eating much or at all. Since your two little guys are "new", this may be due more to some anxiety about their new home, especially in the case of the shy brother.
You want to help them overcome any fear by associating yourself with good things - like food. Bring them some special treats between meals too. Little "rewards" like this for letting you give them some gentle pets, or coming out to play with a wand toy, for example, will encourage them to continue to do so.
Keep a close eye on the brother you suspect is not eating, or not eating much. He could indeed have a health issue. If you have any doubt, better to rule that out with a check-up by the vet.
BTW, congratulations on your adoption of these two lucky kitties! Please keep us updated on how they're doing.