Tracking inventory for multi-varient products

[Ozcorp_Web]

We sell clothing (Bikini), the costumers can choose the size of both the top and the bottom piece of the suit, the hard part is keeping track of the inventory, shopify understands each product as 9 variants (S/S, S/M, S/L, M/S, etc.), Is there any app or way to avoid using the default inventory tracking for this and still allowing costumers to choose freely between sizes?.

 CURRENT INVENTORY

STORE FRONT

Any suggestions are greatly appreciated  !!!!!