You should be able to keep them together, pigeons are extremely social and many breeders and rescuers I know will allow younger and older babies together, they help each other out emotionally and can keep one another warm. Hopefully the younger one is old enough to have gotten some crop milk, which will help the little one in the long run to be healthy and strong. Pigeons are one of the few species of birds that produce a milk-like substance that is highly nutrient dense, which they feed to their babies. That said, you'll want to pick up some baby bird feeding formula, like the Kaytee Exact hand-feeding formula, from a pet store and mix that up. Pigeon babies stick their head into their parents mouth, not the other way around, so you'll want to keep that in mind. I think someone posted a good video on how to work with this.
If you cannot find the formula, then get some baby chick starter, nonmedicated, and soak it in warm water so that you can turn it into a sort of pudding-like substance, and then give that to both of the babies. It isn't ideal, but it is a lot better than just giving them peas (though peas are very nutritious, they seem to be a bit addicting and should not be the soul source of nurtients). You'll want to get chicken or game bird mash/crumbles to feed them as they get older. As for water, make sure you have a dish that is wide and somewhat deep. Pigeons need to be able to fully stick their beak in the water to drink, since they use suction to drink and not gravity, like other birds.
Pigeons, thankfully, grow very rapidly and will be able to fly by 4 weeks old. However, they also imprint quite strongly and as you are raising them there may be concerns about releasing them, as unfortunately a lot of folks are quite mean to pigeons and will harm them if the pigeon does not know to fly away. If possible, I would check some local directories and see if there are any pigeon breeders in your area who might be willing to take in the birds, or if you'd even be willing to keep them. Pigeons that we have in the USA are all feral pigeons, descendants of escaped pets and racing birds. This means they're all domesticated, and as a result rescued squabs do quite well in captivity.
((Or, if you are possibly in the Upstate NY area, we are experienced in handrearing baby birds as well as caring for pigeons so we'd be happy to help, but I doubt you are, haha)).