Need advice on how to structure a 2 property deal

[Eric Stengel]

I am looking at 2 duplex properties next door to eachother owned by the same person. Both properties are listed for $450,000. I would buy one property and house hack, but the other property would need to be sold as an investment and would require 25% down. I don't have enough for the down payment and closing costs on both, but a friend of mine is open to teaming up on the investment and fronting most or all of the down payment for the 2nd one. 

What I don't know is what a fair % split of the deal would be, or what ways I could structure the deal to pay him back or buy him out in a few years. 

Any and all advice is welcome!