i agree with wyld, the two focal points distract the view. after doing lots of research for the aquascaping on planted tanks and reading up on the iwagumi style, i learned that its best to have a main focal point which attracts the viewer and then leads them down a path to view the rest of tank. also, be sure your focal point is not dead center or off to one side of the tank. there's a ratio that aquascapers follow of 1:1:6 on exactly where your focal point should be. you can do a google search on that if you wanna get really technical. i would put those two large pieces together, then maybe add a small third piece to keep the arrangment in odd numbers as this appears more natural.
after many tries, this is what i ended up with. i didnt follow any strict rules but i think the overall layout is good and its in scale with the tank. the focal point is on the left and it slowly leads the eye down to right and the rest of tank. now i just have to get it planted!!