Interesting and creative strategy NI, but getting into a JV investment with your REA means that his investment debt will count against your credit score in future. You do realise that, don't you?
For example, if you owe, say, $200K each on a shared property, his $200K will be regarded as your debt by the bank when you next got for a loan. It's not fair, but that's the way it works (because from the bank's point of view you are a guarantor to his side of the loan).
Happy to be corrected of course if there is a way to get around this with two self-contained loans, but I've not heard of this happening myself. The same thing bit me firmly on the backside when I went into partnership with my sister to buy a place. Talk about forced equity savings: We were both hamstrung for 3 years (until I managed to buy her out)!
You have me thinking... Tar
BUT I must say, getting the REA on side to share in the deal means it must be a good deal (surely), and that you're looking at all the angles to find a way to buy it. Nice going. If it's so good a deal though, why not just go it alone? (The other thing is that the REA would have to disclose to the vendor his interest as purchaser, which might not be in the deal's best interests.)