I can't see how two texts that differ by ~1500 years (in the case of the old testament and the q'uran) and ~500 (new testament and q'uran) can be such "similar texts".

Truth is, they aren't of course, and that if you weren't using (like they are doing in the site) relatively modern translations of the originals that bring the two texts a big together, it would be immediate to see which was which.

All in all and putting it simply, they were written in very different periods which means that they were written by very different people of very different cultures.