DFSM + 1 stack = DPDA
NFSM + 1 stack = NPDA
and we know power(NPDA)>power(DPDA)
DFSM + 2 stack = DTM
NFSM + 2 stack = NTM
and power(DTM) = power(NTM)
even adding more stack to TM does not improve expressing power. ie. I can say a FSM with n stack is same powerful than a FSM with 2 stack.