That's a really good question - I suppose if you want to claim it as a deduction it makes more sense to host in a different account. I'm just starting to learn about taxes and deductions, etc. and it seems to me the more documentation you have the better - since it's for two different business entities I'd keep the info sep. as well.
BUT - since there's no cost for hosting the second site it wouldn't really matter I guess, as there would be no relative expense deduction. (am I making sense??

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We have all of our own sites wrapped up in two servers but we also have all of our business ventures wrapped up in the one LLP company.