I want to be sure I understand your question. You want to make a rectangle from two triangles such that the rectangle is twice as high as it is wide, and the triangles are "made" by drawing a line from one corner to the opposite corner, diagonally. Or half of a TriRec block.
If you know the finished size that you want, you need to add 1.25" to the height and 0.75" to the width. Cut this size rectangle, then cut on the diagonal. For example if you want 4" high by 2" wide finished rectangle, cut rectangles that are 5.25" by 2.75". This will allow for the seam allowance just like when we add 7/8" for half-square triangles. Be sure to sew, so that you only remove 1/4" from the front side, therefore a scant 1/4" on the back side. Otherwise, they will be too small, like many people have problems with the half-square triangles.
The challenge will be in aligning the points for this particular shape and not having them shifted one way or the other.
I would highly recommend playing with Margaret Miller's Angle Play. She has already figured out the angle so that when you place the two pieces together, you line them up and sew. There will not be a long thin point to put into the machine and there is no guess work on alignment. The link above is for videos which she has on her website to show how easy they are.
