I'm not sure I understand your situation. If you mean 2nd mtg secured by current prin dwelling to make a down pmt on another property, then yes it is rescindable. (any ln subject to Reg Z & sec by the equity in the current prin dwell is subject to the right of recission)
If the customer is purchasing a home and at the same time he will be getting a second mortgage on the home he is purchasing for the down payment there are no rescission rights. The purpose of both transactions is to purchase the principal dwelling; thus they are both exempt.
Opinions are my own and not of my employer.