Yes it's possible, the STMIs that route to the SBC need to know where they are routing, you can configure a single IP on each (unless you use DNS SRV), so if active-active SBC you need 2 STMIs (or vSTMIs). If active/standby and SBCs are colocated the SBCc can use a virtual IP in which case you need 1 STMI as there is logically only 1 destination. I've not seen that but there is no reason it won't work. However 2 active SBCs is the usual install.