I once spliced 2 limbs from bows that broke together. One was a ELB style and the other was a flatbow. So, no, they don't have to be sisters. Just make sure you match the characteristics like ring thickness, earlywood:latewood ratio, and density as closely as possible.
If you have 2"x2", you should certainly flatten the belly, making sure it is parallel to the back, or as close as you can get it. It will make life so much easier for you.
With 40" long, you have a decent margin of error if you screw the first cut up. Or even the second...
Yes, you can back it if you want.