Given there are two companies where one is a sales only company and the other one holds all the items in stock. This implies that all sales orders will result in the creation of the intercompany chain and intercompany sales orders. If the reservation is manual for the sales company and automatic in the stock company and the sales quantity that is synced to the intercompany sales order exceeds the quantity that can be reserved in the stock company, there is no warning shown or "Autoreservation" form triggered in the sales order within the sales company.

For regular sales orders inside the stock company there is such a function available to tells the user to either change the product/quantity or acknowledge the lack of items in stock. This should be available within the sales company as well, since the order management and completion is always done within the original sales order.

Autoreservation form: https://imgur.com/a/QEAQsH3