Hi – the fact that you’ve both been operating your businesses independently for a number of years helps. HMRC try to prevent new or existing businesses avoiding VAT registration by artificially splitting into two smaller ones, and they’ll then investigate how “independent” the two businesses really are. But in your case, HMRC presumably wouldn’t currently question whether your businesses were truly independent – so you just need to be sure that nothing suggests they’re being merged.
I can see why you’ve considered the option of renting a separate office space, but I don’t think that would necessary prove anything. It wouldn’t in itself prove the independence of the two businesses, as a single business could operate from multiple locations – and you can often get numerous totally unrelated small businesses operating from a shared workspace. Just continue to keep the ownership and practical operation of the two businesses separate. If there are ever any transactions between them (e.g. an advert on one website for the other’s products?), ensure that they are done on proper commercial terms – with the appropriate documentation and charges being invoiced and paid in the businesses accounts. Effectively, make sure that you both treat each other’s business in the same way that you would treat any other competitor business. If the only change that happens to the two businesses is their owners sharing an address, you should be fine.