First deal under contract and I have private lender question

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Hello fine BP folks,

I am pleased to say that after many months of education and a few offers not accepted, I finally have a property under contract.  It looks like a colleague of mine will invest in the deal.  If he is not on the mortgage, what kind of agreement do we need to paper between us?   Does this need to be done with my real estate attorney?  Our deal is pretty simple 50/50 split but not sure how to draft the deal.  Any advice much appreciated.