Yes you can, as long as you keep the line activeSo if I purchase an iPhone XS max on an existing line and then create a new line for my daughter and purchase an iPhone XS max for that line, I assume I will get the $700 credit.
Question is will I be able to later on switch the XS max bought with the new line to a different line on the account. The new line would stay open with a different phone assigned to it. Do you think I would lose the credit in this situation?