You can do it with the control panel, if it supports it. Unless the control panel or server isn't stable, there would be no problem doing it that way. Alternately, you can use a 3rd party DNS solution - which most domain registrars have available.
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Depends on what panel you use... The main idea is to add an A record that will determine that subdomain.domain.com will point to the ip of host B that was assinged to you. If you give us some more information such as if your host A and B are using any panels and how you have everything set up as of now.