My question is, can Knights of the Sacred Seal use the Soul Binding feature to bind to multiple Vestiges at once as long as one of them is their Patron Vestige?
So Knight of the Sacred Seal states that it stacks with Binder for continuing progression for your Soul Binding feature such as gaining the ability to bind to multiple Vestiges at once. However in the flavor text for it, it says that KotSS can bind other Vestiges but that they choose not to. Likewise in the text for the Patron Vestige feature it points out how you lose access to all supernatural class features until you can bind to your Patron again, but can bind to other Vestiges as normally until then.
you can bind other Vestiges in the interim, as normal for the Binder class.
As such both the fluff and the crunch infer that you can only bind to other Vestiges as a KotSS while waiting to bind to your Patron Vestige again.