worried, my main stem split at the y were it was topped while LST

and now you have a bigger pipeline giving her food. You will notice a knockle start to develop over time and that is like putting a bigger pipe and pump in. You will really like the buds coming from that stalk.

I know this is an old thread, I just wanted to mention that cable ties work perfectly for mending split main stems, they hold it place securely. Mesh bandage tape is even better in general because it allows air to get to the wound so it can breathe while it heals, but for main stems I would use the cable ties.