In web parlance, the word "redirect" has specific meaning. Since an IP address is not needed for a redirect, I assume you are using the term conceptually. Are you splitting hosting between two ISPs?
When a browser sends a request to a web server, it includes a header which contains the domain name you are trying to access. This is why multiple domains can be on a single IP address.
While you can do what your other ISP is asking you to do, keep in mind the IP address may change.
If your site is moved to a new server, or DH moves the server to another data center, or changes the IP address for any other reason, your site will be "broken" until you figure out the address changed, change it in the other ISP's DNS server, and wait for the new address to propagate.
You can have as many domains as you want (to pay for) in one account. Having more than one (billing) account is actually against the DH rules. You can have more than one user account in a billing account.
I could say more about these things, but it is probably better to digest this first, and then ask follow up questions.