For X3-UlTRA inverter, can the two battery strings share one BMS?

No, if you want to connect battery to both of the two groups, two BMS are needed, because for this inverter, the two battery groups are working separately, each group should connect one BMS to manage the batteries connected. Also for the two battery groups, you can connect with differernt type of battery, for example: group1 connect T30, group2 connect T58 V2.