Spell Matrix objects can only hold one matrix, but that matrix can be split up.
Spell Matrix Objects were originally described in Magic: A Manual of Mystic Secrets (FASA 6201), page 39, where it says:
To use any spell matrix object, a magician must first attach a thread to the item. Matrix objects can only have one thread attached at any one time. If a character attaches a thread to a matrix object belonging to someone else, the original thread is displaced and any matrices contained in the item are wiped clean.
Since only one thread can be woven to the object, only one spell matrix can be contained within it as the rank of the thread woven to the spell matrix object determines the Circle of spells it can hold. However they can be split:
If desired the owner of a spell matrix object can "split" the rating
of the attached thread, giving the item several matrices of lesser
Thus if the matrix object owner has woven a Rank 6 thread to their matrix object this can be split into a Rank 3, a rank 2 and a rank 1 matrix. This is chosen each time the Rank of the thread woven to the object is increased.
So if Bob the Elementalist has a Rank 2 thread woven to their Matrix object staff they can, when they weave a Rank 3 thread to it, chose to either increase the existing matrix to Rank 3 or add an additional Rank 1 matrix (Giving the staff a rank 2 and a rank 1 matrix)