Once you create a new quote for i.e. two additional contract lines, via the contract screen, quote tab, for an already existing contract id, it will create a new contract id with both contract lines. It won’t be added to the existing contract. 

The new quote will have a reference to the existing contract, but the quote_contract_lines will reserve a contract id number.

Kevin Lobracco wasn’t sure either, if this is to be considered a bug or requires an enhancement.

This capability is a requirement for an sales case I am working on.

Looking forward second opinions regarding this topic.