I would go with a "Y" setup with a cleanout so it can be used now and later on for different purposes.
if I was setting it up, and was planing on using it as a regual drain or somthing later I would install one of THESE under the ground level a bit then bring the two points up a little bit apart from each other using a 45 degree elbow for the one then cap them both with THIS . this way if you wanted to and plan acordingly with your spacing you could put a toilte into the one that goes strait down and use the other for sink drains or what ever. it could be redundent but for the extra 20ish bucks its worth it just incase you want to use it haha.
I don't like using PVC outside, although many do, and the ABS is a much more heavy duty product.
