I would strongly recommend getting a couple of books by Vicky Halls, Cat Detective and Cat Confidential. She's one of the UK's top cat behaviourists. The books go into detail about rival cats and measures you can take to solve the problem. They actually inspired me to start studying cat behaviour courses.
From the knowledge I've gained from them, I'll do my best. Please note, I'm not an animal behaviourist!
Ask yourself important questions that could explain what's going on:
Has the aggression always been this way or is this something new?
Did they sleep curled up together as kittens? Do they still do that?
Do they groom each other, does one groom the other or is there no grooming? Has that changed at any point?
How old are they?
What kind of home did they come from?
How old were they when you got them?
Do they go outside?
Do you have any other pets?
Are there other cats coming into the garden or even invading the house?
Has anything changed around the time this started or got worse? From a new baby or a house move, to a new carpet or moving some furniture, cats hate change.
Some of these questions may seem strange but it's to figure out if this is down to boredom, your cats reaching the age of "social maturity", jealousy, aggression due to the presence of rival cats, or any one of a number of reasons why a cat feels it needs to resort to violence.
Please don't ignore this situation as any escalation not only risks injury to your cats, it usually involves your cats peeing, spraying and clawing everywhere in a desperate attempt to make themselves feel safe. It can reduce the submissive cat to a miserable, painful existence, unable to safely eat, drink, sleep or use the litter tray. Once it gets to that stage, the cats usually need to be separated.
If, after reading the books, you can't solve the problem, ask your vet for a referral to an animal behaviourist specialising in cats. Many pet insurers cover this. Any properly qualified behaviourist will accept patients only through vet referral, so they know that physical problems have been ruled out. While I recommend the books for you, in this case, if you can afford to do so, I'd recommend getting the referral straight away without trying to DIY the situation.
The behaviourist will be able to spot what's wrong very quickly, it's much harder to do it yourself.
I'm sorry that this sounds so negative. If the behaviourist can solve the problem quickly, things should be fine. The fact that you're paying attention to the situation is a big plus. I would recommend the books for any cat lover, most of us could learn something from reading them. I hope things get sorted out ASAP.