Based on your description, it sounds like you are looking to dynamically update the fields being displayed based on the value of your Store dropdown. Unfortunately this is not yet possible and would require custom code. For organization, it may be best for now to separate the inventory and price for each store into different collapsible rows in the form layout.
For automating tasks with this separate store data, it should be possible to use conditions in X2Workflow to manipulate the proper inventory and price values, but it would be required that the particular store is selected before these operations take place. For example, when a workflow is triggered, you may want to first set the store as appropriate, then later update the proper field for that store, maybe even in another workflow.