Just want to make sure the OP gets totally accurate information.
1. You can use your domain credits for the 1 year extension that's required (by ICAAN) when transferring registrars.
2. Generally, you can't transfer domain hosting to a promo-coded account. The intention of this is to prevent people from rolling their hosting over from one discounted promo-coded account to another year after year and in the extreme case getting hosting for $22.40 forever.
I suggest contacting DreamHost support and explaining your situation. Since you're not intending to abuse the system to get virtually free hosting forever, they may be able to figure out a reasonable solution for you - particularly since in your situation it sounds like both accounts would remain active and it's accounts from two different people. OTOH, since you can imagine that some people (not you) would find a way to abuse a reasonable but exploitable system, DH may have to protect themselves.
As a totally oddball solution, maybe you can transfer registration but not hosting? So you'd tranfer registration to your new account but keep hosting it on your brother's account? Actually, on second thought, I don't think the DH panel system and database will support this...
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