I understand your frustration about the blaming and negativity but unfortunately, you can't force either of them to see the other in a good light. This is something they have to work out themselves. At this age, it's time for them to take responsibility for their own actions. But while you can't create facts, you can create an atmosphere.
First and foremost, don't compare them. And don't praise one in front of the other hoping you will elicit better behavior. If you set a tone for accepting each one for who they are and there is a tacit, unspoken respect for each of the kids, then you will be modeling the behavior you want them to emulate.
You can also try giving them a role in which they each feel separately and together that they have something important to contribute to the family. You can try something like asking them to prepare a weekend family meal together because you won't be around that day. And then make yourself scarce!
What you want to do is create a climate for cooperation and communication between your children. You can try having a discussion in advance, saying this situation is coming up very often and maybe we can find a way to resolve the problem. But find out from them first if the situation bothers them.
If it does bother them, you can help them by encouraging them to communicate about what's happening, making your role minimal. Don't preach. Let them know you respect each of them enough to know they can modify this behavior.
If the behavior doesn't really bother them, then they may be getting something out of the fact that it bothers you. Ask yourself: What is their relationship like when you are not around? If they get along better when you're not around, then they could be bickering for your benefit. They need to get you involved either because they want your attention or because they are jealous of each other.
If that is the case you can remove yourself from the situation when it happens. In general, I think you need to pull yourself out of the situation a bit. If you can't stand the arguments, leave the room, go out for a walk, visit a neighbor. You may not be able to control their behavior but you can control the impact it has on you. And then they have nothing to gain by continuing to bicker. This takes a tremendous amount of strength, but it's worth it.