It's possible to get both "Wyatt Saved" and "Fergus Saved" in one playthrough, but you have to save your game before you make the choice. Otherwise you'll need to replay the entire first mission through the fortress over again.
When you get to the part with Deathshead, when the large armored behemoth is carrying you, the game will save. Quit out, and copy your save onto either the cloud or a flash drive.
When you go back in, choose the person you DON'T want to live during the game. You can't save both. (Sick choice, I know.)
If you choose to save Wyatt, you'll get armor upgrades throughout the game. If you choose to save Fergus, you'll get health upgrades instead.
Once you've made your decision, some nastiness will happen, and you'll get the appropriate achievement. Quit out to the dashboard, and go back in. This time, choose your HDD (or wherever you saved your game initially, just not where you copied your save to). You'll be back before the choice. This time make the decision you want to live with, and you'll pop the second cheeve as well.