Hello,

1. you have to activate sync ot Sale/purchase orders feature for both of the companies.

Notes: 
-there is no resupply checkbox choice between the stocks of two separate companies.

- Inventory transfers are only applicable amongst warehouses which belonges to the same company.
2. after create purchase order from "comapny A" => it will automatically create sales order in "company B"
Note: you have to predefine reordering rules(to resupply based on some rules) for requested product as-> route : Buy, vendor "company B"

this will create reciept in "company A" after purchase order confirming and delivery order from "company B" after sales order confirming in company B.

Note : this taking into consideration reordering rules for min=0, max=0. you can also define your own stock rules.

Hope this will help.