Sales Order back to back buy + ship from warehouse flow

there's a requirement from my user

ITEM_A, this item is back-to-back enabled
in the warehouse qty is 100
user_A creates SO, ITEM_A, qty is 200, submit it

then user_B process the SO on shipment UI
since the SO qty is larger than onhand qty, user wants this shipment to be fully back to back buy flow

but another case, when the SO is 50, onhand is 100
this SO will take data directly to warehouse

how to achieve this?

also, user wants a flexibility to change the shipment to back to back buy flow on shipment UI

regards,
Andrea