Near impossible imo, the flowerhorn will grow faster than the silver, bully it, and probably eat it. If both are already adult size, unless you have an extremely large tank or pond, it is unlikely either fish will tolerate each other; one of them will probably kill the other. Only two ways I see this happening:
1) if you have several tanks set-up (55-75g, 125g, 240g, etc) so that they grow up together and still have a lot of room to swim around as they age. This method allows both fish to become comfortable with each other (although it still doesn't guarantee either fish will not try to kill each other).
2) The alternative is to have a pond or an aquarium at least 1000g (imo) and place both fishes in there (regardless of age) so they have plenty of running space when either get feisty. Just note, it is hard to feed small fish in large tanks and ponds.
