Life with sons is busy! My boys are much older now and still fight over stuff. One thing that worked when they were little was to take the object away from both until they were ready to share or just later when it was forgotten. Even 9 month old babies understand when you tell them to leave big brother's toys alone. And the baby being talked to the same way you talk to the older one will make the older one much happier.
You may need to rethink the toilet on your own plan though. As unbelievable as it may seem, brothers can't be left unattended even for a bit. I couldn't really leave my sons unattended at all - even alone when the older ones were at school - because they were so active and just don't have the judgment needed to stay safe. Our playpen was only for those occasions when I really needed the baby safe - to drain boiling water or use the bathroom or clean up older brother. It is a busy, crazy time but take lots of pictures because they grow up fast.