@lazyrunnr welcome! That’s a good question, and to answer it we have to go into the details of how the envelope budget works. I haven’t tried to run multiple budgets at once, but I’ve been using the envelope budget for a while and I think I understand it.
The main thing is that the Envelope Budget works differently than the default Tiller Foundation template. If you’ve used the Foundation template before you know that the categories sheet is where the budget information is stored, in the columns to the right. On the Envelope Budget sheet the budget information is instead stored on the Budgets History sheet (hidden by default). The categories sheet is only for setting categories, which is why it can be shared by multiple budgets.
Hope that makes sense, feel free to ask more questions!
Also, I have no idea what your situation is, but since you’re reading about envelope budgeting in Tiller you may also be interested in the new Savings budget sheet here: Add the Savings Budget sheet
If you’re just getting started with envelope budgeting in Tiller it is a newer (and in my opinion easier to use) way of doing it. It also uses the Foundation template, which is nice.