On Aruba 2930M - first thing first - you don't for a Backplane Stack by using "normal" DAC Cables but, instead, you must use special Stacking Cables (they look similar to DAC but they haven't the typical expected SFP+ form factor).
So...on a two Members Backplane Stack you can use both Stacking Ports concurrently (each Stacking Module has 2 Stacking Ports) and you can do it by linking any-to-any:
SW1 Stack Module Port 1 <--> Port 1 Stack Module SW2
SW1 Stack Module Port 2 <--> Port 2 Stack Module SW2
or:
SW1 Stack Module Port 1 <--> Port 2 Stack Module SW2
SW1 Stack Module Port 2 <--> Port 1 Stack Module SW2
in any case you're forming a Ring Topology.
It's clear that if you have Three Aruba 2930M you will be forced to a Chain Topology where:
SW1 Stack Module Port 1 <--> Port 2 Stack Module SW2
SW2 Stack Module Port 1 <--> Port 2 Stack Module SW3
SW3 Stack Module Port 1 <--> Port 2 Stack Module SW1 (which closes the loop into SW1)
or:
SW1 Stack Module Port 1 <--> Port 1 Stack Module SW2
SW2 Stack Module Port 2 <--> Port 2 Stack Module SW3
SW3 Stack Module Port 1 <--> Port 2 Stack Module SW1 (which closes the loop into SW1)