You may continue to bind additional vestiges as per a normal binder of your EBL (Effective Binder Level)
While there is no mention of binding other vestiges in the fluff, I think it's a matter of perspective on the fluff. It focuses on how much better a Knight is when binding their patron, but makes no mention of this binding being exclusive.
Either way a strict RAW reading:
Soul Binding: At each knight of the sacred seal level, your soul binding ability improves as if you had also gained a level in the binder class. Thus, your knight of the sacred seal levels and binder levels stack for the purpose of determining your bonus on binding checks, the effectiveness of your vestige-granted abilities, your ability to bind higher-level vestiges, and the number of vestiges you can bind. You do not, however, gain any other benefit a binder would have gained.
Patron Vestige (Su): When you become a knight of the sacred seal, you select any one vestige that you have previously bound as your patron. You develop a stronger than normal relationship with this vestige that grants you several benefits as you advance in this prestige class. If you expel your patron vestige by means of the Expel Vestige feat or do not renew your pact with it on any given day, you lose access to all supernatural class features gained from this prestige class until such time as you contact that vestige again. You can bind other vestiges in the interim, as normal for the binder class, but you gain no additional benefits from those pacts. You are never at risk of making a poor pact with your patron vestige.
Emphasis mine. "As normal for the binder class" no further restrictions.
You may bind any Vestiges that a binder may normally make a pact with. You simply receive additional benefits if it's your Patron.
Your question also seems to imply that you lose access to all supernatural class features when not binding your Patron Vestige. While you are still able to bind any Vestige you meet the required EBL for - even ones that are not your Patron - you do lose access to some features while not binding your selected patron, these are only the Supernatural (SU) abilities granted specifically by the Scared Seal Knight.
Patron Vestige (Su): ... If you expel your patron vestige by means of the Expel Vestige feat or do not renew your pact with it on any given day, you lose access to all supernatural class features gained from this prestige class until such time as you contact that vestige again.
Loses access to:
- Vestige's Protection (Su)
- Vestige's Protection Aura (Su)
- Vestige's Power (Su)
- Apotheosis (Su)
- Vestige Surge (Su)
and technically Patron Vestige itself, but you regain it as soon as you meet the requirements of binding your Patron again.
You still keep:
- All previously gained abilities by the Binder class (and all other classes and PrCs)
- Soul Binding Progression (-) Thus keeping your full EBL at all times.
- Aligned Strike (Ex)
Follow by example
If you are still up in the air, even the example Seal Knight has two vestiges bound, which is appropriate for her EBL 9
Example Knight of Sacred Seal
On page 58, Kybrin (Female dwarf binder7/Knight of Sacred Seal2)
Vestiges Typically Bound (EBL 9th, binding check 1d20+11) Agares, Buer
SQ: .. Patron vestige (agares), soul binding (2 vestiges, 4th level)..
In addition the two other 'Focus on a specific vestige classes' in the tome of magic use similar wording (despite the Tenebrous Apostates's fluff) and both also bind two vestiges in their examples (Their patron and one additional)
Breven Hulstoff on page 63 and Luxx on Page 67
Granted 3.5 example characters aren't always the most reliable, but I feel the rules and the fluff are lining up correctly.
Being able to bind several Vestiges at higher levels is important to EBL classes because they would start to fall very behind other classes. If this is a ruling your DM is imposing, ask them to reconsider.
The only limitation on Binding vestiges I can find at all, are in regards to Amon, Chupoclops, Eurynome, Karsus, Leraje, Primus, and Tenebrous. Each of which will not bind to a Binding-Class if they have bound some combination of the mentioned in the last 24 hours. A few others have some very specific cases too, but are generally easily avoided. These are all usually noted in the Special Requirements for binding.
And as KRyan pointed out, if this is an issue for you at all. There is always the Ignore Special Requirements feat.