I am going to have to disagree with the majority on this one, I would definatly prefer a 9 holer with am/pro tees that could be expanded later into an 18. I think you would be doubling you work to start off with making 2 fairways for each hole and 2 tees for each hole when eventually half of that work is going to be wasted. I would suggest putting that extra work into planning the full 18 ahead of time even though only 9 will be put in to start with. That way your course will a more continuous flow and there will be less work/confussion in the long run while also making your course better overall.
Not only that, but I think having 2 tees throwing at the same pin would slow down the course to the point that it would not get the play that it would if it a true 9 hole course. It would turn groups of 4 into groups of 8 when throwing at holes when the course was full.
IMHO, a hybrid 18 is not worth it and would only cause more greif in the long term.