Jeff, In Steve Shappells book, A Timber Framer's Workshop, he also discusses spline joinery. Being a novice framer I also used four spline joints with diminished haunches in my frame. The splines are 2x5 oak. The next time I would use the offset tenons as discussed by Rudy because of it's ease of cutting and simplicity. What I have learned is, Stick to the basics! Spline joinery takes a lot more time to cut vs two mortices and two tenons.
John..