Thanks for your response.
I have received an okay from the board of the second nonprofit and a copy of the tax determination letter to set up the account, so I won't have to develop the new site as an add-on domain.
My next question is since the nonprofit has two domains registered at another registrar, I plan to use one domain on DH for development. Can I do this and later have the DNS servers configured to point to DH's servers after the site is ready to go live (in about a month or two), then configure both domains DNS servers to point to the same site. I don't have control of the domains yet.