Initial Appointment of Trustees

I am dealing with an estate where the will appoints ”… A, B, C and D to be the executors and trustees of this Will.”

Then, much later in the will, there’s an additional clause which states:

“ “my Trustees” means the persons appointed as executors and trustees under clause 1 and one partner from the firm of XYZ.” “

I know that the purported appointment of one partner or any two partners as executors will be void for uncertainty, but does the same apply with the appointment of trustees?