I agree with fantailer :/ That tank is pretty ridiculous even for a goldfish. Did you know that one goldfish needs a 20 gallon tank to thrive and grow as big as it's supposed to? And to even attempt keeping a koi with it is... please give that koi to someone with a pond! You are
wrong if you think you can keep an adult goldfish and an adult KOI CARP in such a tin can of a tank :/
Even if they have "enough room for now" doesn't mean they'll be healthy! They still produce waste, which is the main reason they need a certain amount of water not to poison themselves. If you want to keep them together so bad, build a good and proper pond!
But since these two fish don't get along, I'd say it's pretty useless to force them to be in the same tank/pond :/
Final answer: Give the koi to the pet shop or someone you know that has a pond, and keep the goldfish. If you'd be extra nice to your goldie, you'd get him a bigger tank