Right first of all that is not my Discus and Angel tank, I do not own one such tank. That was just an example I found to demonstrate it had been done. However, I don't think your views on lighting/planting are completely correct. When Discus first came into the hobby, the only fish that could be kept were wild caught. Now, generation after generation of tank-bred discus fish has lead to a hardier fish that are more commonly used to high lighting levels, tanks surronded by levels of high activity, and even being housed in local tap water.
You say that
discus need plants if there is lights on.
Not true, alot of breeders and specalist keepers will keep their fish in bare bottom tanks with no furniture, this often enables Discus to find food better and often leads to larger specimens. If you wish to keep plants however, you will need bright lighting to a certain level or the plants won't flourish. I do not like it when people claim that Discus are such delicate fish, yes they're are delicate and should be treated with care but they are very capable of living in a well planted tank with high lighting if they have been tank bred for generations and hundreds of people probably keep them in just a way. Wild caught Discus are a different matter ofcourse and ideally they should be provided with a low-level lighting tank in which the water contains the tanins of bogwood etc making it the famous tea colour.
You also say
They will be fine together and why not they come from the same part of the world!
You do know that in the wild Angels and Discus do not co-excist together don't you? Discus originate from different areas of South America, they live in shaded waters that are heavily leeched with tanins from leaves and wood creating the tea colour, because of this there is little or no plant life that can survive. The water is also of a higher temperature and the tanins that leech into make it more acidic creating the tanin coloured, soft, warm and acidic water they are renound for coming from.
Angels originate elsewhere in South America from still or slow-moving streams and river tributaries of the Amazon Basin and Guyana River. These streams and tributaries are often covered with reeds and algae. They are also found in flooded areas, such waters are filled with plant flora unlike the Discus fishes home.