I only split my boys when the dominant poorly one got so wobbly his friend started to push him around - I saw it happening. My boys were happy next door to each other - they were separated by grid bars and they could still see, touch and smell each other through them. But Harvey was protected from Casper and was pretty relieved about that. If you haven't actually seen your boys fighting or squabbling could it be that his injury might just have been caused by catching his foot on something? Or does he get chased around by his friend. Some people who split boys see that the pushy pig will try to get back into the other's space but the picked-on pig ignores them and seems happier in their own space.
You've got an old guy there so you must've been doing a great job looking after him. For the next phase of his life, as he comes to the end eventually, it would be best if he knows his familiar friend is still there with him. If your other pig is much younger you can think about his future when the time comes. And if they are a similar great age, well... who knows what might happen. Well done on finding a vet and getting pain management for him