you could probably do either one.

the advantages of having two separate triangles and keeping your real middle hairline, is that the front center hairline is the most important part of the piece, in that it is the most exposed to various light and angles. keeping your real hair would def look the most natural in that part.

however. disadvantages of two triangles is that you have to line it up to fit the hairline, and match it as well as you can. I never tried this, I think its doable, but im not sure if it would be harder or easier to do. I wouldnnt know till i tried.

regards