It does not sound all that simple.
Are you also executing an asset acquisition agreement protecting you against their pre purchase liabilities such as taxes, debts and potential lawsuits and claims?
Does it include an agreement by the seller of the business to relinquish all right title and interest not only to all names used by the business but any potentially similar or misleading names and also an enforceable covenant by the sellers not to compete?
These are only a couple of concerns that come to mind that are dealt with in the transaction of a business.
The easy part of it is that both of the trustees or co trustees must sign in their capacity as trustees, you must have the signed consent of all beneficiaries, and if you have not seen a copy of the entire trust to determine if there has been compliance with all terms of the trust in the sale, then you need a certificat of trust.
For these and other reasons I highly recommend that you have an experienced business attorney help you with this.
This answer is offered for informational purposes only. It is not offered as, and does not constitute, legal advice. Laws vary widely from state to state. You should rely only on the advice given to you during a personal consultation by a local attorney who is thoroughly familiar with state laws and the area of practice in which your concern lies.