Best way to split an existing drain pipe
From a code perspective it's hard to say because they vary quite a bit. For specific details about that you could always make an anonymous call to your local municipality. On to practical stuff, you can do what you have proposed. Pipe is measured by the inside diameter That being said your pipe is inch and a half. For the rubber couplings, I always bring a piece of the pipe in with me and physically make sure that it fits. If you can get these Fernco couplings, they work well Always Clean, chamfer and deburr your pipe. If you find 8 inches is not enough space you could always cut further up and just pick up an ABS 45.