The best practice here is to aggregate all data into one system, then sync that one system with StockTrim.
For example, if you have multiple e-commerce stores and use Xero for your accounting, you'd sync all stores into your one Xero account, then sync that with StockTrim.
Or, if you have multiple POS accounts (for example 3 different Vend accounts) and use a system like Cin7 to manage your inventory, you'd sync all 3 Vend accounts with Cin7, then sync that one Cin7 account with StockTrim.
Generally speaking it can be awkward to maintain data in different sources (ie: multiple Shopify accounts for products, or having one system for inventory and another one for sales data). So syncing all the data into one source is best practice to do anyway.