You must have a taken king weapon rare or legendary of the same class ( primary, special and heavy). The way it works you must choose which taken King weapon you would like to infuse. Usually it's a legendary due to there unique perks. If you have a 200 base damage legendary weapon and you would like to infuse it to boost up its damage you must choose a rare or legendary weapon That has a base damage above the current weapon you are trying to infuse.So you can infuse a 200 damage weapon with any weapon of the same class that has a higher damage of 200. Infusing will allow the weapon to have a higher damage. Same thing goes for armor, it must be the same class and higher defense. Of course the higher the damage the higher the infuse weapon will become. Usually you will also need glimmer, more of light, weapon parts or armor and 3 legendary marks. I hope this helps you out.

Watch the first stream.

Do both weapon have to have the infuse symbol?

If a weapon is able to be infused, it will have an infuse symbol in the upgrade tree. To actually infuse the weapon, you need weapons of higher damage and legendary marks (3) as well as a few other materials as well (glimmer, motes, weapon/armor parts). This is useful especially if you find rare weapons that have a higher damage value than legendary weapons yet you'd still rather use the legendary weapon as opposed to the rare.

Self bump.