If you took the quest from Scorch in the Gate of Secrets, you will need to travel west in the Domain of Secrets till you find the two bosses next to each other (one a Warrior the other a Mesmer). However, if you took this follow up after you completed "A Flickering Flame" then the bosses can be found a short walk south from where you defeated Ignis Cruor. They can be pulled seperately - a good way to do this is to stand on the hill with the Shrine on it and use a Longbow. Note these titans spawn next to each other, and do not break down into lesser titans. Once you kill one you can safely move on to the other.