>>Is it possible allocate back-to-back sales order lines?

The user should be able to set it to normal and allocate it.   The PO direct quantity can influence this choice.  

Take a look at the PO direct quantity field to somewhat automate the process maybe.  

In the event of a purchase back-to-back order, the system will determine that it is a direct back-to-back order (the supplier delivers the goods to the customer) when the quantity ordered is greater than the direct quantity (expressed in the product sales units).   Field help (CTMQTY2) This field contains the quantity (expressed in sales units) from which the direct order will be straightforward.

There is probably a way to set this or modify it to have this CTMQTY2 be dynamic based on product in stock.   Then if it could be filled from stock it would and if not it would default to back to back.  

Thanks Stephen , but if the way is a development (to fill the field), it is not an option in this moment.

I will try to configure as default the more common case, and the user change the rest of cases in the order.

>>but if the way is a development (to fill the field)

It works as it is but it's a fixed number.   So you have to say if the order is for a quantity greater than say 5, drop ship it.   Otherwise ship from stock.    

The customization would be to change this to look at stock available and say, if i have enough to cover ship it and if i don't drop ship it.    If you set it to 1, it should always choose to drop ship it.   If i was doing the modification i would add a field by item that said something like "Consider Stock before selecting B to B."    Then a combination of the PO direct Quantity and consider stock could be used to decide.    The PO direct quantity is handy so that you don't wipe out your stock with a large order.

Thanks Stephen. Finally, the btb will be manual over the sales order.