Brothers from the same litter can live together, if they have to. As long as they grow up together there's a chance that they never start fighting, but that's not guaranteed.
Friends of mine had 2 brothers living in their home for about 10 years without incidents. Then one of the brothers got urinary tract stones and started soiling the house and furniture. That prompted the second one to start fighting and try and chase his brother away (probably because of his soiling). They had to keep both cats separated to avoid fights and ended up giving one cat away to a family member.
You should really think about neutering your cats. You write
I'd like them to have 'normal' cat lives...
A neutered cat has a perfectly normal cat life. He even has a more relaxed cat life than an intact male cat.
- Intact cats mark their territories, which stinks, especially if they start marking at the walls of your home. Neutered cats mark less often and it doesn't stink as much.
- Intact cats get restless during mating season and may stay away from home for up to 3 days in search of a mate. Neutered cats don't feel the urge to mate (although they are physically still capable) and are much more relaxed.
- Both cats are more likely to tolerate each other if at least one of them is neutered.
- If they roam free (or escape your house) they may mate with a feral cat, increasing the problem of malnourished feral cats with poor health that are hard to tame and find homes for.