Invoking more than one demon for magic spels

As far as i have been told Bune would be the best for magic spells. But i want to invoke more than one demon.Do i have to concentrate on the sigils in the same time or one at a time?
Thank you

It’s actually best to just work with one demon at a time for this kind of thing… But if you still are, concentrate on the sigils one at a time and open them individually