I apologize. I thought I had written up an answer but apparently did not.
Reading up on the BenQ W710ST it's native spec is 1280x720 16:9 aspect ratio with a variable lens throw ratio of between 0.719:1 and 0.79:1. My primary concern with your design is that it's important to use the projector at native resolution for the best results. You make mention of a 4:3 aspect ratio instead of 16:9.
Most of my initial delay in getting back with you was that I'm not used to working in metric numbers. The general rule of design i use is not to go over 60" projection height on the screen design with a 720p projector. Also not to exceed 110 degrees field of view (including blending area) per projector. That tends to work itself out to a screen area that is between 5 foot and 7.5 foot radius for a 0.72:1 dual projection setup.
You make mention of using an AMD 7750. The 7750 should be capable of Eyefinity 2x1, so no need for the Matrox DH2G box unless you need to run more than just the external view from that video card. I'm a bit weary of pushing 2560x720 from a 7750 for FSX, as FSX gets really bogged down fast without a top of the line video card. It'll work, but it won't be high FPS. You'll need to optimize the fsx.cfg file to offset the FPS rate.
The 7750 will work and the BenQ 710ST will also work, but they're not ideal. My personal preference would be to use at least 1280x800 capable projector and a bit more powerful video card. At least with the video card you can always pick up a more powerful card later on should the 7750 prove insufficient. My ideal pick of projector right now would be the BenQ W1080ST. There isn't anything else on the market in that has it's resolution and short throw lens combination. I'm uncertain as to what your current budget is and if you have any of the hardware already though. I really would suggest saving up for a more powerful video card and wait till you can afford the W1080ST's. There is no compromise that way in your build. As a project like this is something that is a large expenditure of time and money I know there is a want to cut costs somewhere, but I'd be weary on cutting it on the video card and the projectors. Either way we can still get you working with the BenQ 710ST's and the AMD 7750 if it's the budget limitation. Be mindful that all 7000-series AMD cards are capable of Eyefinity, so there is no need to puchase a Matrox DH2G box, so that'll save you some money.