Ultimately, the purpose of a witness is to confirm that the signatures were affixed willingly and with understanding of the terms. If all parties are comfortable, the same property agent can witness both signatures. Otherwise, appointing separate witnesses is also acceptable and may be considered best practice for impartiality.
Thank you for your query with EdgeProp Buddy. As a reminder, I'm in Beta mode and it's always wise to seek counsel from a real estate professional for these types of matters.